You can nest STRUCTURE field description statements.   The  STRUCTURE
  field  ADDRESS,  for  example,  has  a  subordinate  STRUCTURE field,
  ZIP_CODE.

  ADDRESS STRUCTURE.
      STREET                    DATATYPE IS TEXT
                                SIZE IS 30 CHARACTERS.
      CITY                      DATATYPE IS TEXT
                                SIZE IS 30 CHARACTERS.
      STATE                     DATATYPE IS TEXT
                                SIZE IS 2 CHARACTERS.
      ZIP_CODE STRUCTURE.
          NEW                   DATATYPE IS UNSIGNED NUMERIC
                                SIZE IS 4 DIGITS
                                BLANK WHEN ZERO.
          OLD                   DATATYPE IS UNSIGNED NUMERIC
                                SIZE IS 5 DIGITS.
      END ZIP_CODE STRUCTURE.
  END ADDRESS STRUCTURE.