x versus _ as x ????

Hi everyone,

I created my own little riffle function.  It's similar ( but not exactly
the same as ) Wolfram Mathematica's Riffle function.  Mathematica's Riffle
output is limited by the length of the smaller list.  My riffle output's
length is determined by the length of the larger list.  A small difference
really.  Anyhow, here is my function:

let riffle list1 list2 = let add_two_lists l1 l2 l3 =
                                    match l1, l2 with
                                    |x, [ ] -> l3 @ x
                                    |[ ], y -> l3 @ y
                                    |h1 :: t1, h2 :: t2 -> h1 :: h2 ::
add_two_lists t1 t2 l3 in
                                    add_two_lists list1 list2 [ ] ;;

It works great!  It works exactly as I want it to, BUT HERE'S THE PROBLEM.
The following function DOES NOT EVEN COMPILE!!!!  Why????

*let riffle list1 list2 = let add_two_lists l1 l2 l3 = *
*                                    match l1, l2 with *
*                                    |_ as x, [ ] -> l3 @ x*
*                                    |[ ], _ as y -> l3 @ y*
*                                    |h1 :: t1, h2 :: t2 -> h1 :: h2 ::
add_two_lists t1 t2 l3 in *
*                                    add_two_lists list1 list2 [ ] ;;*

Perhaps I do not fully understand how to use the "as" keyword, but I
thought that the two functions should compile and produce the exact same
output.  What's the difference between the two forms?  I appreciate the
feedback!

Best,

Douglas Lewit.

P.S.  Can someone provide a simple example of how to initialize and use a
Map data structure?