Evaluating atoms

An atom is either

• a predicate atom (a predicate applied to expressions, like even(N)), or
• a comparison of two expressions (=, !=, <, >, <=, >=).

They differ in how they are evaluated:

• A predicate atom is matched against that predicate’s relation: variables are bound to the matching values, while constants and the don’t-care _ only filter.
  • num(N) → N ∈ {1, 2, 3, 4, 5, 6}
  • even(N) → N ∈ {2, 4, 6}
  • divisor(_, 6) succeeds, divisor(_, 5) fails; _ and the constant bind nothing
• A comparison is a test on already-bound values: D > 1 holds exactly when D exceeds 1.