That's funny. I forgot let is a special form in CL. It doesn't really need to be, and it isn't in Arc.

  (mac with (parms . body)
    `((fn ,(map1 car (pair parms))
       ,@body)
      ,@(map1 cadr (pair parms))))
      
  (mac let (var val . body)
    `(with (,var ,val) ,@body))