Logical negation.

Short-circuit conjunction: if the first argument is False, the second is never evaluated (Lazy).

Short-circuit disjunction: if the first argument is True, the second is never evaluated (Lazy).

Force a lazy value. In practice, oppi evaluates Lazy arguments only when the value is actually used.

Conditional: the unused branch is never evaluated (Lazy). Used by the if/then/else notation below.

Always true — the "else" guard of guarded definitions:

abs n | n < 0 = -n | otherwise = n