Changed RX line

Old:

# RX   MEDLINE; 91002678.
RX = Martel.Group("RX",
                  Martel.Str("RX   ") + 
                  Martel.Word("bibliographic_database_name") +
                  Martel.Str("; ") +
                  Martel.Digits("bibliographic_identifier") + 
                  Martel.Str(".") +
                  Martel.AnyEol())
New:
# RX   MEDLINE=93305731; PubMed=7916637;
bib = (Martel.Word("bibliographic_database_name") + Martel.Str("=") +
       Martel.ToSep("bibliographic_identifier", ";")
       )
RX_exp = (Martel.Str("RX   ") + bib +
          Martel.Opt(Martel.Str(" ") + bib) +
          Martel.AnyEol())
 
.