in (unify) working on ref-containment detection to avoid cycle creation
return :fail if attempt to unify would create a cyclic graph.
whitespace, comments and logging
remove workaround for cycles: revert to existing "per" and "a" prepositions: do not specify :sen :pred for either of these.
in (overc), check for comp being nil; if so, return nil.
fix typo
increase some logging in overc from TRACE to DEBUG
redefine test lexicon and grammar while working on cycle detection