cleanup: restrict what's required to make it more clear what's being used in here.
simplified :comment values
less verbose logging
add more cond cases to handle partially-realized phrases
extract 

(defn parents-at-this-depth [head phrases depth])

from 

(defn lightning-bolt [head phrases depth lexicon])
using empty? rather than .size to avoid realization of lazy seqs
remove redundant cond case in (fo-ps)
more fixes for printing out surface forms
debugging instrumentation
debugging instrumentation: add minil (miniature lexicon) and minip (miniature phrase set)
less verbose logging: don't log over and overc unification fails at INFO (only at DEBUG).
less verbose