equiv_conversion_impl
=====================

A conversion from a localist\_pattern's keyed_representation to another
representation, or a string\_pattern to another string\_pattern, that seeks to
maintain the content of the pattern.  In the localist\_pattern case, the 
dimension
name of every present element is used to generate a target representation pattern,
and these are superposed.  In the string\_pattern case, a copy is made, and the
new representation imposed.