proposal to extend rhs / lhs to all relational operators, maybe others

Hello,

At present rhs and lhs return the right-hand side and left-hand side,
respectively, of an expression of the form a = b.
For any operator other than "=", rhs returns 0 while lhs returns the
whole expression.

I would like to change that so that rhs and lhs return the
right- and left-hand sides for all relational operators
(namely < <= = equal # notequal >= and >).

We might also consider having rhs / lhs recognize := ::= : and ::
since those also have unambiguous right- and left-hand sides.
Maybe there are others.

It is true that the right- and left-hand sides can be obtained at present
via part. However, (1) the operands of all relational operators should
be accessible the same way -- either all via rhs / lhs or none of them.
(2) There is precedent for having general-purpose named part functions
(e.g. op, args) as well as part itself.

I ran into this when I was trying to work with some inequalities.
Not a big deal, but it seems obvious and the implementation is simple
(see patch below).

Comments?

All the best,
Robert Dodier

PS.
Index: src/csimp.lisp
===================================================================
RCS file: /cvsroot/maxima/maxima/src/csimp.lisp,v
retrieving revision 1.6
diff -u -r1.6 csimp.lisp
--- src/csimp.lisp  7 Nov 2005 17:37:11 -0000   1.6
+++ src/csimp.lisp  8 Jan 2006 19:26:30 -0000
@@ -173,11 +173,21 @@

 (defun subin (y x) (cond ((not (among var x)) x) (t
(maxima-substitute y var x))))

-(defmfun $rhs (eq)
-  (cond ((or (atom eq) (not (eq (caar eq) 'mequal))) 0) (t (caddr eq))))
-
-(defmfun $lhs (eq)
-  (cond ((or (atom eq) (not (eq (caar eq) 'mequal))) eq) (t (cadr eq))))
+(let
+  ((relops '(mlessp mleqp mequal mnotequal $equal $notequal mgeqp mgreaterp
+                    %mlessp %mleqp %mequal %mnotequal %equal
%notequal %mgeqp %mgreaterp)))
+  (defmfun $rhs (rel)
+     (if (atom rel)
+       0
+       (if (memq (caar rel) relops)
+         (caddr rel)
+         0)))
+  (defmfun $lhs (rel)
+     (if (atom rel)
+       rel
+       (if (memq (caar rel) relops)
+         (cadr rel)
+         rel))))

 (defun ratgreaterp (x y)
   (cond ((and (mnump x) (mnump y))