- pre_context(Name, Goal, Var) is semidet[multifile, wordlist(trace)]
- post_context(Name, Goal, Var) is semidet[multifile, wordlist(trace)]
- Multifile hooks to extract additional information from the
Pengine, either just before Goal is started or after an answer
was produced. Extracting the information is triggered by
introducing a variable with a reserved name.