instead of constructing simply lazy seq of parents with heads, construct a lazy seq of maps of parents to parents with heads.
rename map-lexicon to get-lexicon-of-heads
cleanup
add 2 new params to get-parents-with-phrasal-head-map: path-to-here (logging) and lexicon-of-heads (memorized subset of lexicon)
logging: blank path-to-here delimiters
throw exception if rat-hole detected - no possible comp-phrases for head.