If you consider the Common Lisp spec, both assoc and member provide a test predicate for searching.
This compliments the various tests for equality which uLisp expects elsewhere.
Off the top of my head we’ve got eq, equal, and string=. People could provide a custom test in the case they wanted to compare structures which were equal in some other higher order way. For example if you had instances with a unique id but only wanted to compare their “type” contained in a record for equality.