query_context
    type:       $SCA_QUERY_CONTEXT
    access:     read/write
    mechanism:  by reference

    An SCA query context that is to describe the indicated query
    expression.

 query_expression_string
    type:       character string
    access:     read only
    mechanism:  by descriptor

    A query expression string.

 query_expression_length
    type:       longword
    access:     write only
    mechanism:  by reference

    Optional. Length of the query expression, returned from the
    parser.