Enhanced Substitution to parse the lookUp field, handle compound lookUps and
magic values, setting several predicates for interrogation, adding some error
handling, and then using the changes to greatly clarify the parsing logic in
i_processLookUp(..). This also exposes some more functionality directly to
unit testing. Added testcases. Probably getting to the bottom of the refactoring
for this pass. Mucked with doc comments to hopefully make the structure more
clear.