Queries

A query is a nameless rule, so it translates the same way. Asking what is reachable from one station:

?- reach("Taipei Main Station", Y).

The constant in column #1 is a filter; the variable Y in column #2 is what we keep:

π_#2( σ_{#1 = "Taipei Main Station"}( reach ) )

Two goals that share all their variables intersect: the handout's ?- prime(N), even(N) is prime ∩ even. Everything that held for rule bodies holds for queries.