Segment attributes illegally redefined

A SEGMENT directive re-opens a segment that has been previously defined,
and tries to give it different attributes. For example,

   SEGMENT data BYTE PUBLIC
   ENDS
   SEGMENT data PARA ;error, previously had byte
                     ;  alignment 
   ENDS

If you re-open a segment, the attributes you supply must either match
exactly or be omitted entirely. If you don't supply any attributes when
re-opening a segment, the old attributes will be used.