I recently wrote some ocaml code which "worked", but not as I 
intended...  The test cases I tried worked, but I should have tested 
harder.  Apparently I was under the mistaken impression that OCaml's 
pattern-matching was more "first class"!  So I wrote (in part):

let buildsimp cast e f1 f2 = fun e1 -> fun e2 -> match (e1,e2) with
                                                 | ({st = Some e}, _) -> e2

and I expected it to work.  Only a code review by a colleague 'found' 
this bug in my code.

Question: would it be a difficult extension?  This seemed so "natural", 
I just "used" the feature before it was quite there yet ;-).

Jacques

PS: I guess I had first-class patterns on the brain; Maple has them 
[under the misleadingly-named function "typematch"] (but that's 
cheating, I know) and W. Kahl's Pattern Matching Calculus allows it, and 
I wrote a joint paper with Wolfram describing a categorical semantics 
for the PMC.