keyed_representation
====================

A localist representation where each dimension represents a distinct object or 
feature.  Typically comes from string columns in dataframes, or created by
a conversion as its target automatically, rather than configued manually.
Its natural pattern is a localist\_pattern, but it is very helpful when used
with strength\_pattern-based layers.

add_symbol
==========

	some_kr add_symbol sym

Explicitly extend the representation `some_kr` to have the additional dimension
`sym`.

conversion_key
==============

	some_kr conversion_key

Return a key_conversion that converts to a dimension name from the corresponding
unit (basis) vector.

conversion_unkey
==============

	some_kr conversion_unkey

Return a unkey_conversion that converts from a dimension name to a corresponding
unit (basis) vector.