Spec ::= and(Spec,Spec)
Spec ::= or(Spec,Spec)
Spec ::= not(Spec)
Spec ::= sounds(Like)
Spec ::= stem(Like)             % same as stem(Like, en)
Spec ::= stem(Like, Lang)
Spec ::= prefix(Prefix)
Spec ::= between(Low, High)     % Numerical between
Spec ::= ge(High)               % Numerical greater-equal
Spec ::= le(Low)                % Numerical less-equal
Spec ::= Token

sounds(Like) and stem(Like) both map to a disjunction. First we compile the spec to normal form: a disjunction of conjunctions on elementary tokens. Then we execute all the conjunctions and generate the union using ordered-set algorithms.

Stopgaps are ignored. If the final result is only a stopgap, the predicate fails.

To be done

- Exploit ordering of numbers and allow for > N, < N, etc.