Substructure_declarations

 A field within a structure can itself be a structured item composed
 of other fields, other structures, or both.  You can declare a
 substructure in two ways:

  o  By nesting structure declarations within other structure or
     union declarations (with the limitation that you cannot refer
     to a structure inside itself at any level of nesting).

     One or more field names must be defined in the STRUCTURE
     statement for the substructure because all fields in a
     structure must be named.  In this case, the substructure is
     being used as a field within a structure or union.

     Field names within the same declaration nesting level must be
     unique, but an inner structure declaration can include field
     names used in an outer structure declaration without conflict.

     %FILL can be specified in place of a field name to leave space
     in a record for purposes such as alignment.

  o  By using a RECORD statement that specifies another previously
     defined record structure, thereby including it in the structure
     being declared.