[project @ 2001-04-28 11:20:46 by qrczak]

[project @ 2001-04-28 11:20:46 by qrczak]
Fix the bug that source rules of primops shadowed builtin rules: let
extendRuleBase take the old rules from the id passed as the argument
when the id is absent in RuleBase.

I hope this is correct: I don't know what difference can be between
the id passed as extendRuleBase's argument and the id found in RuleBase.

Also use standard isJust, isNothing instead of non-standard
Maybes.maybeToBool.