X =.. Y

Y  is  a list whose head is the atom corresponding to the principal functor
of X and whose tail is the argument list of that functor in X.  eg.:-

       product(0, N, N - 1) =.. [product, 0, N, N - 1]

If  X  is instantiated to a variable, then Y must be instantiated to a list
of determinate length whose head is atomic (ie.  an atom or integer).
