Success: [eclipse 2]: [user]. :- module_interface(m). :- op(700, xf, there). :- export p/1. :- begin_module(m). p(X) :- writeln(X). user compiled 56 bytes in 0.03 seconds yes. [eclipse 3]: p(hello there). syntax error: postfix/infix operator expected | p(hello there). | ^ here [eclipse 3]: use_module(m). yes. [eclipse 4]: p(hello there). hello there yes. Error: begin_module(M). (Error 4). begin_module(1). (Error 5). begin_module(a_locked_module). (Error 82).