Unify Term1 with Term2. True if the unification
succeeds. It acts as if defined by the following fact:
=(Term, Term).
For behaviour on cyclic terms see the Prolog flag
occurs_check.
Calls to =/2 in a clause
body are compiled and may be (re)moved depending on the Prolog flag
optimise_unify.
See also section 2.17.3.