1. - for each keynum in name.key file (each current record)
     - get the history
     - for each preamble in the history
       - seen{ preamble }++ (name.tmp.seen dbm file)
       - write to name.tmp.history
         - transnum keynum status [date reclen user]?
2. - for each record in name.n.dat files (each transaction)
     - die "seen too many times" if seen{ preamble }++ > 1
     - write to name.tmp.transactions
       - transnum keynum status [date reclen user]?
     - if read line from name.md5
       - compare transnum keynum user md5--die if not equal
     - else write to name.md5
       - transnum keynum user md5 (of record data) [date reclen]?

result:
    name.tmp.seen dbm file(s)       - of no use after step 2.
    name.tmp.history flat file      - can compare to new after migrate
                                          of no use after that
    name.tmp.transactions flat file - can compare to new after migrate
                                          of no use after that
    name.md5 [sha]?                 - can compare to new after migrate
                                          keep around for next validation

Note: we plan to add optional 'digest' preamble field for one of
      crc, md5, sha  (of record data)

data scanning procedure:

read each data record in from_ds
  read first preamble
    get reclen, read record, skip recsep
    read next preamble
    repeat until end of file
repeat for every datafile