SWI-Prolog 7.4.2 Reference Manual: Section 6.8

## 6.8 Operators and modules

Operators (section 4.25) are local to modules, where the initial table behaves as if it is copied from the module `user` (see section 6.10). A specific operator can be disabled inside a module using `:- op(0, Type, Name)`. Inheritance from the public table can be restored using `:- op(-1, Type, Name)`.

In addition to using the op/3 directive, operators can be declared in the module/2 directive as shown below. Such operator declarations are visible inside the module, and importing such a module makes the operators visible in the target module. Exporting operators is typically used by modules that implement sub-languages such as chr (see chapter 8). The example below is copied from the library `library(clpfd)`.

    :- module(clpfd,
              [ op(760, yfx, #<==>),
                op(750, xfy, #==>),
                op(750, yfx, #<==),
                op(740, yfx, #\/),
                ...
                (#<==>)/2,
                (#==>)/2,
                (#<==)/2,
                (#\/)/2,
                ...
              ]).