Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 ops_body(Arg1, Arg2, Arg3, Arg4)

 sub(Arg1, Arg2, Arg3)

 stoch(Arg1, Arg2)

 log_prob_dir(Arg1, Arg2, Arg3)

 log_part_dir(Arg1, Arg2)

 mean_log_dir(Arg1, Arg2)