def to_mom( compiler )
      @parfait_block = @block.to_mom(compiler) if @block
      @receiver = SelfExpression.new(compiler.receiver_type) if @receiver.is_a?(SelfExpression)
      yield_call(compiler)
    end
def to_mom( compiler )
      @parfait_block = @block.to_mom(compiler) if @block
      @receiver = SelfExpression.new(compiler.receiver_type) if @receiver.is_a?(SelfExpression)
      yield_call(compiler)
    end
A Yield breaks down to 2 steps:
 - Setting up the next message, with receiver, arguments, and (importantly) return address
 - a SimpleCall,