argument checking, was: About submatrix and its meaning

Richard Fateman <fateman at cs.berkeley.edu> wrote:

   How about
   foo(x,y,??x>y):= ....
   
   so the predicate can involve all the parameters.
   Note  also the possibility of extending this idea to function 
   discrimination based on types or predicates
   
   myabs(x??x>0):=x;
   myabs(x??x<=0):= -x;
   
   which brings us closer to (for example) common lisp object system, and 
   incidentally to Mathematica.

Before you go much further with this ;language extension, I suggest you
think carefully about the operator precedence of this new ?? operator.
There might be unexpected parsing surprises if the test predicates
become more complex.