Robert Smith avatar Robert Smith committed f94737d


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     (make-symbolic-function parameters
                             (list f expression))))
+(defun function-arity (sf)
+  "What is the arity of the symbolic function SF?"
+  (length (symbolic-function.parameters sf)))
 (defun bound-variable-p (var sf)
   "Is the variable VAR bound in the symbolic function SF?"
   (and (member var (symbolic-function.parameters sf) :test 'eq)
 (defun nullary-function-p (sf)
   "Is the symbolic function SF nullary?"
-  (null (symbolic-function.parameters sf)))
+  (zerop (function-arity sf)))
 (defun unary-function-p (sf)
-  "Is the symbolic function SF nullary?"
-  (= 1 (length (symbolic-function.parameters sf))))
+  "Is the symbolic function SF unary?"
+  (= 1 (function-arity sf)))
+(defun binary-function-p (sf)
+  "Is the symbolic function SF binary?"
+  (= 2 (function-arity sf)))
 ;;; XXX: This should probably make a copy/new function.
 (defun alpha-convert (sf new-var &optional old-var)
          (t (error "Don't know how to diff ~S" expr))))))
 (defun differentiate (sf)
-  (assert (= 1 (length (symbolic-function.parameters sf)))
+  (assert (unary-function-p sf)
           "Symbolic function must have exactly one parameter.")
   (let ((var (first (symbolic-function.parameters sf)))
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