ENH unfinished functional tag stripper
ENH delete empty nodes, strip function tags
ENH force rst_dt.ptb to output clean trees
MAINT minor code improvements after kowey review
Merge branch 'master' into ctree-remove-functional-tags
ENH is_empty_category
MAINT pep8 and pylint
FIX filter empty tokens after dirty tweaking
Merge pull request #14 from moreymat/ctree-remove-functional-tags

Clean PTB trees: remove empty nodes, strip functional tags