.NH
UNIFYING PRINCIPAL FUNCTORS
.LP
If UNSW Prolog was compiled with PRINC_VARS defined then the following
is allowed:
.DS I
f(1).

g(X, Y) :- X(Y).

g(f, X)?

X = 1
.DE
The principal functor of a compound term may be a variable.
Prolog will perform unification on the principal functor in the same
way is it unifies the arguments of a term.
Note that `X' must be bound before a goal such as `X(Y)' can be satisfied.
