Sets the line number of the first line in the <B>TextB> widget. The
default <B>-toindexcmdB> and <B>-fromindexcmdB> use this configuration
-item Name: <B>toIndexCmdB> <B>fromIndexCmdB>
-item Class: <B>ToIndexCmdB> <B>FromIndexCmdB>
-item Switch: <B>-toindexcmdB> <B>-fromindexcmdB>
These two options specify callbacks that are called with a list of
indexes and are responsible for translating them to/from indexes that
the base <B>TextB> widget can understand. The callback is passed the
widget followed by a list of indexes, and should return a list of
translated indexes. <B>-toindexcmdB> should translate from user
indexes to native <B>TextB>-compatible indexes, and <B>-fromindexcmdB>
should translate from native indexes to user indexes.
The default callbacks simply add/subtract the offset given by the
<B>-linestartB> option for all indexes in line.character format.
It would probably be prudent to make these functions inverses of each