SYNOPSIS
	mixed assoc(mixed key, mixed *keys, mixed *data_or_fail, mixed fail);

DESCRIPTION
	Searches a key in an alist.

	Three modes of calling:
	i ) With exactly two arguments, the second being an array
	which's first element is no array.
	In this case the entire array is searched for the key; -1 is
	returned if not found, else the index (like member_array(), but
	faster on sorted arrays).

	ii) With two or three arguments, the second being an array
	which's first element is an array.
	The array has to have a second element of the same size; the
	key is searched in the first and the associated element of the
	second array that is element of second argument is returned if
	succesful; if not, 0 is returned, or the third argument, if
	given.

	iii) With three or four arguments, the second being an array
	of keys (first element no array) and the second is a
	matching data array. Returns 0 or fourth argument (if given)
	for failure, or the matching entry in the array given as third
	argument for success.

	Complexity : O( lg(n) ) , where n is the number of keys.

	Return value is undefined if another list is given in place of
	a presorted key list.

SEE ALSO
	alists(LPC), order_alist(E), insert_alist(E)