Looks up item in association list taking default in case of failure.
DESCRIPTION
The call assocd x [x1,y1; ...; xn,yn] y returns the first yi in the list
where the corresponding xi is the same as x. If there is no such item, it
returns the value y. This is similar to assoc except that the latter will
fail rather than take a default.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# assocd 2 [1,2; 2,4; 3,6] (-1);;
val it : int = 4
# assocd 4 [1,2; 2,4; 3,6] (-1);;
val it : int = -1