To add or remove an operator, use op(Precedence,
Type, Name). op/3 declares the atom Name
to be an operator of the stated Type and Precedence. If
Precedence is 0, the operator properties of Name
(if any) are cancelled.
Please note: operators are global, as opposed to being
local to the current module, Prolog text, or otherwise.
See mpg-ref-op.
To examine the set of operators currently in force, use
current_op(Precedence, Type, Name).
See mpg-ref-current_op.