MEMBER.html -- ACL2 Version 6.3

### MEMBER

membership predicate

    Major Section:  ACL2-BUILT-INS

    General Forms:
    (member x lst)
    (member x lst :test 'eql)   ; same as above (eql as equality test)
    (member x lst :test 'eq)    ; same, but eq is equality test
    (member x lst :test 'equal) ; same, but equal is equality test

`(Member x lst)` equals the longest tail of the list `lst` that begins with `x`, or else `nil` if no such tail exists. The optional keyword, `:TEST`, has no effect logically, but provides the test (default `eql`) used for comparing `x` with successive elements of `lst`.

The guard for a call of `member` depends on the test. In all cases, the second argument must satisfy `true-listp`. If the test is `eql`, then either the first argument must be suitable for `eql` (see eqlablep) or the second argument must satisfy `eqlable-listp`. If the test is `eq`, then either the first argument must be a symbol or the second argument must satisfy `symbol-listp`.

See equality-variants for a discussion of the relation between `member` and its variants:

> `(member-eq x lst)` is equivalent to `(member x lst :test 'eq)`;

> `(member-equal x lst)` is equivalent to `(member x lst :test 'equal)`.

In particular, reasoning about any of these primitives reduces to reasoning about the function `member-equal`.

`Member` is defined by Common Lisp. See any Common Lisp documentation for more information.