;; ANSI says this function "should signal an error if [..] argument
;; is not a float". Since X is otherwise ignored, Python doesn't
;; check the type by default, so we have to do it ourself:
;; ANSI says this function "should signal an error if [..] argument
;; is not a float". Since X is otherwise ignored, Python doesn't
;; check the type by default, so we have to do it ourself: