Check mmodule to avoid a new instantiation of ASTBuilder

Property definitions

nitc $ ASTBuilder :: check_mmodule
	# Check mmodule to avoid a new instantiation of ASTBuilder
	fun check_mmodule(mmodule: MModule)
	do
		if self.mmodule != mmodule then self.mmodule = mmodule
	end
src/astbuilder.nit:34,2--38,4