A special form of the FOR statement sets up a record stream
    consisting of segments from a segmented string field. Because
    a single segmented string field value is made up of multiple
    segments, a record stream that includes a segmented string field
    is "nested." The outer loop retrieves records that include the
    field and the inner loop retrieves the segments of each field
    value one at a time. Therefore, a FOR statement that retrieves
    segmented strings looks like a set of nested FOR statements.

    Example:

    RDO>  FOR R IN RESUMES
    cont>    FOR S IN R.RESUME
    cont>    PRINT S.RDB$LENGTH, S.RDB$VALUE
    cont>    END_FOR
    cont>  END_FOR