Regula S-Expressions don't enforce assumption about MANY cardinality

[tealeg]

There is an assumption in the regula/rule/operator.operator handling of Contracts. We assume that only the final Term in the contract may have Cardinality == MANY. We should enforce that rule somehow - I'm not yet sure how, but it seems a panic would be the best we can expect (I can't see a way to have this fail at compile time).