assoc-default is a compiled Lisp function in `subr.el'.

(assoc-default KEY ALIST &optional TEST DEFAULT)

Find object KEY in a pseudo-alist ALIST.
ALIST is a list of conses or objects. Each element
(or the element's car, if it is a cons) is compared with KEY by
calling TEST, with two arguments: (i) the element or its car,
and (ii) KEY.
If that is non-nil, the element matches; then `assoc-default'
returns the element's cdr, if it is a cons, or DEFAULT if the
element is not a cons.

If no element matches, the value is nil.
If TEST is omitted or nil, `equal' is used.