in                           Test membership

    The  in  keyword  allows  the  user  to  test  membership  in arrays in
    expressions.  The form of an expression containing the in keyword is:

                                expression in A

    if the value of 'expression' is a current subscript value of the  array
    A,  the  expression  yields  a  TRUE  value,  otherwise  FALSE.     For
    multidimensional arrays, the statement:

                             expression in A[i]

    would test if 'expression' is a  valid column subscript in the ith  row
    of array  A. Note  that A  may have  more than  two dimensions for this
    statement to be correct.  The next higher dimension than stated in  the
    expression is always tested.