+;;; result makes sense. It will if F is monotonic increasing (or, if
+;;; the interval is closed, non-decreasing).
+;;;
+;;; (Actually most uses of INTERVAL-FUNC are coercions to float types,
+;;; which are not monotonic increasing, so default to calling
+;;; BOUND-FUNC with a non-strict argument).
+(defun interval-func (f x &optional increasing)