spatial_representation
======================

A spatial representation in the sense of Davis's SCM, associated with
spatial_pattern.

set_keys
========

	some_spr set_keys key_rep...

Set the dimensions of `some_spr` to be the same as those of `key_rep`,
a `keyed_representation`, and associate `key_rep` with this
representation (e.g., for the purposes of setting up a 
`blocked_localist_to_spatial_conversion`.

key_rep
=======

	some_spr key_rep

Return the `keyed_representation` set with `set_keys`.