The trace mode is switched on and set to leap.
When tracing, a leap command to the debugger will cause execution to continue until it reaches a spied predicate. Setting a spy-point on a non-existing procedure results in an error.
Note that spypoints can also be set using set_flag(Proc, spy, on).
Success: [eclipse]: [user]. lower:-writeln(hi). higher:-lower. user compiled 84 bytes in 0.00 seconds yes. [eclipse]: spy higher,writeln. spypoint added to higher / 0. spypoint added to writeln / 1. spypoint added to writeln / 2. yes. [eclipse]: higher. +(1) 0 CALL higher (dbg)?- leap B+(3) 2 CALL writeln(hi) (dbg)?- leap hi B+(3) 2 EXIT writeln(hi) (dbg)?- leap +(1) 0 EXIT higher (dbg)?- leap yes. Error: spy(I). (Error 4). spy("l"). (Error 5). spy(foo). (Error 60).