# Evaluate `n` as an expression in the current context.
	# Return the value of the expression.
	# If `n` cannot be evaluated, then aborts.
	fun expr(n: AExpr): nullable Instance
	do
		var frame = self.frame
		var old = frame.current_node
		frame.current_node = n
		#n.debug("IN Execute expr")
		var i = n.expr(self)
		if i == null and not self.is_escaping then
			n.debug("inconsitance: no value and not escaping.")
		end
		var implicit_cast_to = n.implicit_cast_to
		if i != null and implicit_cast_to != null then
			var mtype = self.unanchor_type(implicit_cast_to)
			if not self.is_subtype(i.mtype, mtype) then n.fatal(self, "Cast failed. Expected `{implicit_cast_to}`, got `{i.mtype}`")
		end

		#n.debug("OUT Execute expr: value is {i}")
		#if not is_subtype(i.mtype, n.mtype.as(not null)) then n.debug("Expected {n.mtype.as(not null)} got {i}")
		frame.current_node = old
		return i
	end