private fun is_variable_set(variable: Variable): Bool
	do
		if self.set_vars.has(variable) then return true
		var previous = self.previous
		if previous.length == 0 then return false
		if previous.length == 1 then return previous.first.is_variable_set(variable)
		for p in self.previous do
			if not p.is_variable_set(variable) then
				return false
			end
		end
		# Cache the result
		self.set_vars.add(variable)
		return true
	end