\DOC assoc2

\TYPE {assoc2 : ''a -> ('b * ''a) list -> ('b * ''a)option}

\SYNOPSIS
Searches a list of pairs for a pair whose second component equals a specified
value.

\KEYWORDS
list.

\DESCRIBE
An invocation {assoc2 y [(x1,y1),...,(xn,yn)]} returns {SOME (xi,yi)}
for the first {(xi,yi)} in the list such that {yi} equals
{y}. Otherwise, {NONE} is returned. The lookup is done on an eqtype,
i.e., the SML implementation must be able to decide equality for the
type of {y}.

\FAILURE
Never fails.

\EXAMPLE
{
- assoc2 2 [(1,4),(3,2),(2,5),(2,6)];
> val it = SOME (3, 2) : (int * int) option
}


\SEEALSO
Lib.assoc, Lib.assoc1, Lib.rev_assoc, Lib.mem, Lib.tryfind, Lib.exists,
Lib.all.

\ENDDOC