"error"

    my $error = $group->error;

Holds group's error message.

"name"

    $group->name('foo');
    my $name = $group->name;

Group name.

"equal"

Shortcut

    $group->constraint(equal => @_);

"constraint"

    $group->constraint(equal => @_);

Adds constraint to the group.

"is_valid"

Checks whether group's constraints are valid.

"unique"

    $group->constraint(equal => @_);