rel-kets[*]
rel-kets[op1, op2, ..., opn]
If the parameter is *, then return a superposition of all kets that are defined with respect to a literal operator
Else, return a superposition of all kets that are defined with respect to the given literal operators
Very useful for generating input lists of objects of interest.
Note, kets that are only on the RHS of a learn rule are not included in the rel-kets superpositions
If this is desired, then one approach is to invoke find-inverse[op] first,
so that now they are on the LHS of a learn rule.