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lfe_guide(7) lfe_guide(7)

lfe_guide - Lisp Flavoured Erlang User Guide

Note: {{ ... }} is use to denote optional syntax.

Integers can be written in various forms and number bases:
Regular decimal notation:
  1234 -123 0
    
Binary notation:
  #b0 #b10101 #b-1100
    
Binary notation (alternative form):
  #*0 #b*10101 #*-1100
    
Octal notation:
  #o377 #o-111
    
Explicitly decimal notation:
  #d1234 #d-123 #d0
    
Hexadecimal notation:
  #xc0ffe 0x-01
    
Notation with explicit base (up to 36):
  #2r1010 #8r377 #36rhelloworld
    
Character notation (the value is the Unicode code point of the character):
  #\a #\$ #\ä
    
Character notation with the value in hexadecimal:
  #\x1f42d;
    
In all these forms, the case of the indicating letter is not significant, i.e. #b1010 and #B1010 are identical as are #16rf00 and #16Rf00.
Similarly, the case is not significant for digits beyond 9 (i.e. 'a', 'b', 'c', ... for number bases larger than 10), e.g. #xabcd is the same as #xABCD and can even be mixed in the same number, e.g. #36rHelloWorld is valid and the same number as #36Rhelloworld and #36rHELLOWORLD.
The character notation using hexadecimal code representation (#\x....;) is basically the same thing as the regular hexadecimal notation #x... except that it conveys to the reader that a character is intended and that it does a sanity check on the value (e.g. negative numbers and value outside the Unicode range are not permitted).

There is only one type of floating point numbers and the literals are written in the usual way, e.g. these are all valid floating point numbers:
1.0 +1.0 -1.0 1.0e10 1.111e-10
    
The one thing to watch out for is that you cannot omit the the part before or after the decimal point if it is zero. E.g. the following are not valid forms: 100. or .125.

There are two forms of strings: list strings and binary strings.

List strings are just lists of integers (where the values have to be from a certain set of numbers that are considered valid characters) but they have their own syntax for literals (which will also be used for integer lists as an output representation if the list contents looks like it is meant to be a string): "any text between double quotes where " and other special characters like \n can be escaped".
As a special case you can also write out the character number in the form \xHHH; (where "HHH" is an integer in hexadecimal notation), e.g. "\x61;\x62;\x63;" is a complicated way of writing "abc". This can be convenient when writing Unicode letters not easily typeable or viewable with regular fonts. E.g. "Cat: \\x1f639;" might be easier to type (and view on output devices without a Unicode font) then typing the actual unicode letter.

Binary strings are just like list strings but they are represented differently in the virtual machine. The simple syntax is #"...", e.g. #"This is a binary string \n with some \"escaped\" and quoted (\\x1f639;) characters"
You can also use the general format for creating binaries (#B(...), described below), e.g. #B("a"), #"a", and #B(97) are all the same binary string.

Certain control characters can be more readably included by using their escaped name:
  | Escaped name | Character       |
  |--------------+-----------------|
  | \b           | Backspace       |
  | \t           | Tab             |
  | \n           | Newline         |
  | \v           | Vertical tab    |
  | \f           | Form Feed       |
  | \r           | Carriage Return |
  | \e           | Escape          |
  | \s           | Space           |
  | \d           | Delete          |
    
Alternatively you can also use the hexadecimal character encoding, e.g. "a\nb" and "a\x0a;b" are the same string.

We have already seen binary strings, but the #B(...) syntax can be used to create binaries with any contents. Unless the contents is a simple integer you need to annotate it with a type and/or size.
Example invocations are that show the various annotations:
> #B(42 (42 (size 16)) (42 (size 32)))
#B(42 0 42 0 0 0 42)
> #B(-42 111 (-42 (size 16)) 111 (-42 (size 32)))
#B(-42 111 (-42 (size 16)) 111 (-42 (size 32)))
> #B((42 (size 32) big-endian) (42 (size 32) little-endian))
#B(0 0 0 42 42 0 0 0)
> #B((1.23 float) (1.23 (size 32) float) (1.23 (size 64) float))
#B(63 243 174 20 122 225 71 174 63 157 112 164 63 243 174 20
   122 225 71 174)
> #B((#"a" binary) (#"b" binary))
#"ab"
    
Learn more about "segments" of binary data e.g. in "Learn You Some Erlang (http://learnyousomeerlang.com/starting-out-for-real#bit-syntax)" <http://learnyousomeerlang.com/starting-out-for-real#bit-syntax>.

Lists are formed either as ( ... ) or [ ... ] where the optional elements of the list are separated by some form or whitespace. For example:
()
(the empty list)
(foo bar baz)
(foo
 bar
 baz)
    

Tuples are written as #(value1 value2 ...). The empty tuple #() is also valid.

Maps are written as #M(key1 value1 key2 value2 ...) The empty map is also valid and written as #M().

Things that cannot be parsed as any of the above are usually considered as a symbol.
Simple examples are foo, Foo, foo-bar, :foo. But also somewhat surprisingly 123foo and 1.23e4extra (but note that illegal digits don't make a number a symbol when using the explicit number base notation, e.g. #b10foo gives an error).
Symbol names can contain a surprising breadth or characters, basically all of the latin-1 character set without control character, whitespace, the various brackets, double quotes and semicolon.
Of these, only |, \', ', ,, and # may not be the first character of the symbol's name (but they are allowed as subsequent letters).
I.e. these are all legal symbols: foo, foo, µ#, ±1, 451°F.
Symbols can be explicitly constructed by wrapping their name in vertical bars, e.g. |foo|, |symbol name with spaces|. In this case the name can contain any character of in the range from 0 to 255 (or even none, i.e. || is a valid symbol). The vertical bar in the symbol name needs to be escaped: |symbol with a vertical bar \| in its name| (similarly you will obviously have to escape the escape character as well).

Comments come in two forms: line comments and block comments.
Line comments start with a semicolon (;) and finish with the end of the line.
Block comments are written as #| comment text |# where the comment text may span multiple lines but my not contain another block comment, i.e. it may not contain the character sequence #|.

#.(... some expression ...). E.g. #.(+ 1 1) will evaluate the (+ 1 1) while it reads the expression and then be effectively 2.

(quote e)
(cons head tail)
(car e)
(cdr e)
(list e ... )
(tuple e ... )
(tref tuple index)
(tset tuple index val)
(binary seg ... )
(map key val ...)
(map-get m k) (map-set m k v ...) (map-update m k v ...)
(lambda (arg ...) ...)
(match-lambda
  ((arg ... ) {{(when e ...)}} ...)           - Matches clauses
  ... )
(function func-name arity)                    - Function references
(function mod-name func-name arity)
(let ((pat {{(when e ...)}} e)
      ...)
  ... )
(let-function ((name lambda|match-lambda)     - Local functions
               ... )
  ... )
(letrec-function ((name lambda|match-lambda)  - Local functions
                  ... )
  ... )
(let-macro ((name lambda-match-lambda)        - Local macros
            ...)
  ...)
(progn ... )
(if test true-expr {{false-expr}})
(case e
  (pat {{(when e ...)}} ...)
  ... ))
(receive
  (pat {{(when e ...)}} ... )
  ...
  (after timeout ... ))
(catch ... )
(try
  e
  {{(case ((pat {{(when e ...)}} ... )
          ... ))}}
  {{(catch
     (((tuple type value ignore) {{(when e ...)}}
                                - Must be tuple of length 3!
      ... )
     ... )}}
  {{(after ... )}})
(funcall func arg ... )
(call mod func arg ... )        - Call to Mod:Func(Arg, ... )
(define-module name meta-data attributes) (extend-module meta-data attributes)
(define-function name meta-data lambda|match-lambda) (define-macro name meta-data lambda|match-lambda)

(: mod func arg ... ) =>
        (call 'mod 'func arg ... )
(mod:func arg ... ) =>
        (call 'mod 'func arg ... )
(? {{timeout {{default}} }})
(++ ... )
(list* ...)
(let* (...) ... )
(flet ((name (arg ...) {{doc-string}} ...)
       ...)
  ...)
(flet* (...) ... )
(fletrec ((name (arg ...) {{doc-string}} ...)
          ...)
  ...)
(cond ...
      {{(?= pat expr)}}
      ... )
(andalso ... )
(orelse ... )
(fun func arity)
(fun mod func arity)
(lc (qual ...) ...)
(list-comp (qual ...) ...)
(bc (qual ...) ...)
(binary-comp (qual ...) ...)
(match-spec ...)
    

(defun name (arg ...) {{doc-string}} ...)
(defun name
  {{doc-string}}
  ((argpat ...) ...)
  ...)
(defmacro name (arg ...) {{doc-string}} ...)
(defmacro name arg {{doc-string}} ...)
(defmacro name
  {{doc-string}}
  ((argpat ...) ...)
  ...)
(defsyntax name
  (pat exp)
  ...)
(macrolet ((name (arg ...) {{doc-string}} ...)
           ...)
  ...)
(syntaxlet ((name (pat exp) ...)
            ...)
  ...)
(prog1 ...)
(prog2 ...)
(defmodule name ...)
(defrecord name ...)
    

(define (name arg ...) ...)
(define name lambda|match-lambda)
(define-syntax name
  (syntax-rules (pat exp) ...)|(macro (pat body) ...))
(let-syntax ((name ...)
             ...)
  ...)
(begin ...)
(define-record name ...)
    

Written as normal data expressions where symbols are variables and use quote to match explicit values. Binaries and tuples have special syntax.
{ok,X}                  -> (tuple 'ok x)
error                   -> 'error
{yes,[X|Xs]}            -> (tuple 'yes (cons x xs))
<<34,U:16,F/float>>     -> (binary 34 (u (size 16)) (f float))
[P|Ps]=All              -> (= (cons p ps) all)
    
Repeated variables are supported in patterns and there is an automatic comparison of values.
_ as the "don't care" variable is supported. This means that the symbol _, which is a perfectly valid symbol, can never be bound through pattern matching.
Aliases are defined with the (= pattern1 pattern2) pattern. As in Erlang patterns they can be used anywhere in a pattern.
CAVEAT The lint pass of the compiler checks for aliases and if they are possible to match. If not an error is flagged. This is not the best way. Instead there should be a warning and the offending clause removed, but later passes of the compiler can't handle this yet.

Wherever a pattern occurs (in let, case, receive, lc, etc.) it can be followed by an optional guard which has the form (when test ...). Guard tests are the same as in vanilla Erlang and can contain the following guard expressions:
(quote e)
(cons gexpr gexpr)
(car gexpr)
(cdr gexpr)
(list gexpr ...)
(tuple gexpr ...)
(tref gexpr gexpr)
(binary ...)
(progn gtest ...)           - Sequence of guard tests
(if gexpr gexpr gexpr)
(type-test e)
(guard-bif ...)             - Guard BIFs, arithmetic,
                              boolean and comparison operators
    
An empty guard, (when), always succeeds as there is no test which fails. This simplifies writing macros which handle guards.

Inside functions defined with defun LFE permits optional comment strings in the Common Lisp style after the argument list. So we can have:
(defun max (x y)
  "The max function."
  (if (>= x y) x y))
    
Optional comments are also allowed in match style functions after the function name and before the clauses:
(defun max
  "The max function."
  ((x y) (when (>= x y)) x)
  ((x y) y))
    
This is also possible in a similar style in local functions defined by flet and fletrec:
(defun foo (x y)
  "The max function."
  (flet ((m (a b)
           "Local comment."
           (if (>= a b) a b)))
    (m x y)))
    

Variables are lexically scoped and bound by lambda, match-lambda and let forms. All variables which are bound within these forms shadow variables bound outside but other variables occurring in the bodies of these forms will be imported from the surrounding environments.No variables are exported out of the form. So for example the following function:
(defun foo (x y z)
  (let ((x (zip y)))
    (zap x z))
  (zop x y))
    
The variable y in the call (zip y) comes from the function arguments. However, the x bound in the let will shadow the x from the arguments so in the call (zap x z) the x is bound in the let while the z comes from the function arguments. In the final (zop x y) both x and y come from the function arguments as the let does not export x.

Functions are lexically scoped and bound by the top-level defun and by the macros flet and fletrec. LFE is a Lisp-2 so functions and variables have separate namespaces and when searching for function both name and arity are used. This means that when calling a function which has been bound to a variable using (funcall func-var arg ...) is required to call lambda/match-lambda bound to a variable or used as a value.
Unqualified functions shadow as stated above which results in the following order within a module, outermost to innermost:
Predefined Erlang BIFs
Predefined LFE BIFs
Imports
Top-level defines
Flet/fletrec
Core forms, these can never be shadowed
This means that it is perfectly legal to shadow BIFs by imports, BIFs/imports by top-level functions and BIFs/imports/top-level by fletrecs. In this respect there is nothing special about BIfs, they just behave as prefined imported functions, a whopping big (import (from erlang ...)). EXCEPT that we know about guard BIFs and expression BIFs. If you want a private version of spawn then define it, there will be no warnings.
CAVEAT This does not hold for the supported core forms. These can be shadowed by imports or redefined but the compiler will always use the core meaning and never an alternative. Silently!

(defmodule name
  "This is the module documentation."
  (export (f 2) (g 1) ... )
  (export all)                          ;Export all functions
  (import (from mod (f1 2) (f2 1) ... )
          (rename mod ((f1 2) sune) ((f2 1) kurt) ... ))
  (import (prefix mod mod-prefix))      - NYI
  (attr-1 value-1 value-2)
  ... )
    
Can have multiple export and import declarations within module declaration. The (export all) declaration is allowed together with other export declarations and overrides them. Other attributes which are not recognised by the compiler are allowed and are simply passed on to the module and can be accessed through module_info/0-1.

(defmodule (name par1 par2 ... )
  ... )
    
Define a parameterized module which behaves the same way as in vanilla Erlang. For now avoid defining functions 'new' and 'instance'.

Macro calls are expanded in both body and patterns. This can be very useful to have both make and match macros, but be careful with names.
A macro is function of two argument which is a called with a list of the arguments to the macro call and the current macro environment. It can be either a lambda or a match-lambda. The basic forms for defining macros are:
(define-macro name meta-data lambda|match-lambda)
(let-macro ((name lambda|match-lambda)
  ...)
    
Macros are definitely NOT hygienic in any form.
To simplify writing macros there are a number of predefined macros:
(defmacro name (arg ...) ...)
(defmacro name arg ...)
(defmacro name ((argpat ...) body) ...)
    
Defmacro can be used for defining simple macros or sequences of matches depending on whether the arguments are a simple list of symbols or can be interpreted as a list of pattern/body pairs. In the second case when the argument is just a symbol it will be bound to the whole argument list. For example:
(defmacro double (a) `(+ ,a ,a))
(defmacro my-list args `(list ,@args))
(defmacro andalso
  ((list e) `,e)
  ((cons e es) `(if ,e (andalso ,@es) 'false))
  (() `'true))
    
The macro definitions in a macrolet obey the same rules as defmacro.
The macro functions created by defmacro and macrolet automatically add the second argument with the current macro environment with the name $ENV. This allows explicit expansion of macros inside the macro and also manipulation of the macro environment. No changes to the environment are exported outside the macro.
User defined macros shadow the predefined macros so it is possible to redefine the built-in macro definitions. However, see the caveat below!
Yes, we have the backquote. It is implemented as a macro so it is expanded at macro expansion time.
Local functions that are only available at compile time and can be called by macros are defined using eval-when-compile:
(defmacro foo (x)
  ...
  (foo-helper m n)
  ...)
(eval-when-compile   (defun foo-helper (a b)     ...)
  )
There can be many eval-when-compile forms. Functions defined within an eval-when-compile are mutually recursive but they can only call other local functions defined in an earlier eval-when-compile and macros defined earlier in the file. Functions defined in eval-when-compile which are called by macros can defined after the macro but must be defined before the macro is used.
Scheme's syntax rules are an easy way to define macros where the body is just a simple expansion. These are supported with defsyntax and syntaxlet. Note that the patterns are only the arguments to the macro call and do not contain the macro name. So using them we would get:
(defsyntax andalso
  (() 'true)
  ((e) e)
  ((e . es) (case e ('true (andalso . es)) ('false 'false))))
    
N.B. These are definitely NOT hygienic.
CAVEAT While it is perfectly legal to define a Core form as a macro these will silently be ignored by the compiler.

Inside macros defined with defmacro LFE permits optional comment strings in the Common Lisp style after the argument list. So we can have:
(defmacro double (a)
  "Double macro."
  `(+ ,a ,a))
    
Optional comments are also allowed in match style macros after the macro name and before the clauses:
(defmacro my-list args
  "List of arguments."
  `(list ,@args))
(defmacro andalso   "The andalso form."   ((list e) `,e)   ((cons e es) `(if ,e (andalso ,@es) 'false))   (() `'true))
This is also possible in a similar style in local functions defined by macrolet:
(defun foo (x y)
  "The max function."
  (macrolet ((m (a b)
               "Poor macro definition."
               `(if (>= ,a ,b) ,a ,b)))
    (m x y)))
    

Cond has been extended with the extra test (?= pat expr) which tests if the result of expr matches pat. If so it binds the variables in pat which can be used in the cond. A optional guard is allowed here. An example:
(cond ((foo x) ...)
      ((?= (cons x xs) (when (is_atom x)) (bar y))
       (fubar xs (baz x)))
      ((?= (tuple 'ok x) (baz y))
       (zipit x))
      ... )
    

Records are tuples with the record name as first element and the rest of the fields in order exactly like "normal" Erlang records. As with Erlang records the default default value is 'undefined'.
(defrecord name
  field
  (field default-value)
  ... )
    
Will create access functions/macros for creation and accessing fields. The make-, match- and set- forms takes optional argument pairs field-name value to get non-default values. E.g. for
(defrecord person
  (name "")
  (address "")
  age)
    
the following will be generated:
(make-person {{field value}} ... )
 (match-person {{field value}} ... )
 (is-person r)
 (fields-person)
 (emp-person {{field value}} ... )
 (set-person r {{field value}} ... )
 (person-name r)
 (person-name)
 (set-person-name r name)
 (person-age r)
 (person-age)
 (set-person-age r age)
 (person-address r)
 (set-person-address r address)
    
(make-person name "Robert" age 54) - Will create a new person record with the name field set to "Robert", the age field set to 54 and the address field set to the default "".
(match-person name name age 55) - Will match a person with age 55 and bind the variable name to the name field of the record. Can use any variable name here.
(is-person john) - Test if john is a person record.
(emp-person age '$1) - Create an Ets Match Pattern for record person where the age field is set to $1 and all other fields are set to '_.
(person-address john) - Return the address field of the person record john.
(person-address) - Return the index of the address field of a person record.
(set-person-address john "back street") - Sets the address field of the person record john to "back street".
(set-person john age 35 address "front street") - In the person record john set the age field to 35 and the address field to "front street".
(fields-person) - Returns a list of fields for the record. This is useful for when using LFE with Mnesia, as the record field names don't have to be provided manually in the create_table call.
(size-person) - Returns the size of the record tuple.

A binary is
(binary seg ... )
    
where seg is
        byte
        string
        (val integer|float|binary|bitstring|bytes|bits
             (size n) (unit n)
             big-endian|little-endian|native-endian
             big|little|native
             signed|unsigned)
    
val can also be a string in which case the specifiers will be applied to every character in the string. As strings are just lists of integers these are also valid here. In a binary constant all literal forms are allowed on input but they will always be written as bytes.

A map is:
(map key value ... )
    
To access maps there are the following forms:
(map-get map key) - Return the value associated with key in map.
(map-set map key val ... ) - Set keys in map to values.
(map-update map key val ... ) - Update keys in map to values. Note that this form requires all the keys to exist.
N.B. This syntax for processing maps has stablized but may change in the future!
There is also an alternate short form map, mref, mset, mupd based on the Maclisp array reference forms. They take the same arguments as their longer alternatives.

List/binary comprehensions are supported as macros. The syntax for list comprehensions is:
(lc (qual  ...) expr ... )
(list-comp (qual  ...) expr ... )
    
where the final expr is used to generate the elements of the list.
The syntax for binary comprehensions is:
(bc (qual  ...) expr ... )
(binary-comp (qual  ...) expr ... )
    
where the final expr is a bitseg expr and is used to generate the elements of the binary.
The supported qualifiers, in both list/binary comprehensions are:
(<- pat {{guard}} list-expr)        - Extract elements from list
(<= bin-pat {{guard}} binary-expr)  - Extract elements from binary
(?= pat {{guard}} expr)  - Match test and bind variables in pat
expr                     - Normal boolean test
    
Some examples:
(lc ((<- v (when (> v 5)) l1)
     (== (rem v 2) 0))
  v)
    
returns a list of all the even elements of the list l1 which are greater than 5.
(bc ((<= (f float (size 32)) b1)        ;Only bitseg needed
     (> f 10.0))
  (: io fwrite "~p\n" (list f))
  (f float (size 64)))                  ;Only bitseg needed
    
returns a binary of floats of size 64 of floats which are larger than 10.0 from the binary b1 and of size 32. The returned numbers are first printed.
N.B. A word of warning when using guards when extracting elements from a binary. When a match/guard fails for a binary no more attempts will be made to extract data from the binary. This means that even if a value could be extracted from the binary if the guard fails this value will be lost and extraction will cease. This is NOT the same as having following boolean test which may remove an element but will not stop extraction. Using a guard is probably not what you want!
Normal vanilla Erlang does the same thing but does not allow guards.

Apart from (emp-record ...) macros for ETS Match Patterns, which are also valid in Mnesia, LFE also supports match specifications and Query List Comprehensions. The syntax for a match specification is the same as for match-lambdas:
(match-spec
  ((arg ... ) {{(when e ...)}} ...)             - Matches clauses
  ... )
    
For example:
(ets:select db (match-spec
                 ([(tuple _ a b)] (when (> a 3)) (tuple 'ok b))))
    
It is a macro which creates the match specification structure which is used in ets:select and mnesia:select. The same match-spec macro can also be used with the dbg module. The same restrictions as to what can be done apply as for vanilla match specifications:
There is only a limited number of BIFs which are allowed
There are some special functions only for use with dbg
For ets/mnesia it takes a single parameter which must a tuple or a variable
For dbg it takes a single parameter which must a list or a variable
N.B. the current macro neither knows nor cares whether it is being used in ets/mnesia or in dbg. It is up to the user to get this right.
Macros, especially record macros, can freely be used inside match specs.
CAVEAT Some things which are known not to work in the current version are andalso, orelse and record updates.

LFE supports QLCs for mnesia through the qlc macro. It has the same structure as a list comprehension and generates a Query Handle in the same way as with qlc:q([...]). The handle can be used together with all the combination functions in the module qlc.
For example:
(qlc (lc ((<- (tuple k v) (: ets table e2)) (== k i)) v)
     {{Option}})
    
Macros, especially record macros, can freely be used inside query list comprehensions.
CAVEAT Some things which are known not to work in the current version are nested QLCs and let/case/recieve which shadow variables.

The following more or less standard lisp functions are predefined:
(<arith_op> expr ...)
(<comp_op> expr ...)
    
The standard arithmentic operators, + - * /, and comparison operators, > >= < =< == /= =:= =/= , can take multiple arguments the same as their standard lisp counterparts. This is still experimental and implemented using macros. They do, however, behave like normal functions and evaluate ALL their arguments before doing the arithmetic/comparisons operations.
(acons key value list)
(pairlis keys values {{list}})
(assoc key list)
(assoc-if test list)
(assoc-if-not test list)
(rassoc value list)
(rassoc-if test list)
(rassoc-if-not test list)
    
The standard association list functions.
(subst new old tree)
(subst-if new test tree)
(subst-if-not new test tree)
(sublis alist tree)
    
The standard substituition functions.
(macroexpand-1 expr {{environment}})
    
If Expr is a macro call, does one round of expansion, otherwise returns Expr.
(macroexpand expr {{environment}})
    
Returns the expansion returned by calling macroexpand-1 repeatedly, starting with Expr, until the result is no longer a macro call.
(macroexpand-all expr {{environment}})
    
Returns the expansion from the expression where all macro calls have been expanded with macroexpand.
NOTE that when no explicit environment is given the macroexpand functions then only the default built-in macros will be expanded. Inside macros and in the shell the variable $ENV is bound to the current macro environment.
(eval expr {{environment}})
    
Evaluate the expression expr. Note that only the pre-defined lisp functions, erlang BIFs and exported functions can be called. Also no local variables can be accessed. To access local variables the expr to be evaluated can be wrapped in a let defining these.
For example if the data we wish to evaluate is in the variable expr and it assumes there is a local variable "foo" which it needs to access then we could evaluate it by calling:
(eval `(let ((foo ,foo)) ,expr))
    

NYI - Not Yet Implemented
N.B. - Nota bene (note well)

lfe(1), lfescript(1), lfe_cl(3)

Robert Virding.
2008-2016

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