Add subscripts for the Input

[JCumin]

e.g. ?₁ is the second element of ?, ?₎ is the last element of ?, etc.

This can be done by adding predicates calls like constraint variables at the beginning of a rule, one for each ?_i that there is in the rule.

[JCumin]

This is implemented; however this does not work when that subscripted input is in a list, e.g. [?₎]. This needs to be fixed.

[JCumin]

The implementation has been reverted. This is much more tricky to implement than my original naïve implementation. I will get back to making a proper implementation once I can.
(Thanks to @ais523 for discovering that it is bugged)