validate tree for required nodes refactor treegen code
validate tree for required nodes
refactor treegen code

jvasileff/ceylon.language,ceylon/ceylon.language,jvasileff/ceylon.language,unratito/ceylon.language,lucaswerkmeister/ceylon.language,lucaswerkmeister/ceylon.language,unratito/ceylon.language,ceylon/ceylon.language