in                           array membership

 exp in A   test if exp is a subscript of array a
 A          an array variable
 exp        any valid expression

    The in operator tests evaluates the expression 'exp' and tests  whether
    the resultant  value is  a subscript  of the  array variable.   If  the
    variable is an array variable (by virtue of having been referenced with
    a subscript  in a  previous expression)  and 'exp'  matches a subscript
    value, a  value of  TRUE is  returned, otherwise  a value  of FALSE  is
    returned.  A subscript value will not be created.