op(priority, type, name)

Treat  name  as an operator of the stated type and priority.  name may also
be  a list of names in which case all are to be treated as operators of the
stated type and priority.

If  Name is already an operator or list of operators, Priority and Type are
successively bound to the priority and type of Name.

  for eg.

	: op(600, xfx, if)!		% define infix operator

	: op(P, T, infix if)?

	P = 600
	T = xfx
