A spatial representation in the sense of Davis's SCM, associated with spatial_pattern.
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.
some_spr key_rep
Return the keyed_representation set with set_keys.