head-principle also includes :agr (a parent's :agr is the same as the child's :agr)
add shortcut sent-impl; example usage:

(fo (sent-impl (first (take 1 (gen14 seed-phrases lexicon (take 1 (gen14 seed-phrases lexicon lexicon)))))))
gen14 requires an additional argument: a parent's implicature function (i.e. sets infl to one of {:future, :present}.