HELP ISIN                                           A. Sloman, July 1982

    <pattern> isin <list> -> <boolean>

ISIN is an infix operator which takes a pattern and a list and returns <true>
or <false>. The result is true if at least one element of the list matches the
pattern. As a side effect the element of the list which matched the pattern is
assigned to the global variable IT. For example:

    vars x y;
    [?x isa ?y]
      isin
           [[colour a red] [colour b blue] [a isa block] [b isa block]] =>
    ** <true>

    x=>
    ** a

    y=>
    ** block

    it=>
    ** [a isa block]

See also HELP
    *AREIN        - matches a list of patterns against list elements
    *MATCHESONEOF - matches a list against elements of a list of patterns
    *DATABASE     - on use of the POP-11 DATABASE
    *PRESENT      - matches a pattern against the DATABASE
    *FOREACH      - selective iteration over a database
    *MATCHES      - summary of the POP-11 matcher
