Master transaction

open TF and MF
read TF and MF
perform until MF-EOF AND TF-EOF
    if MF-key = TF-KEY
        update record
        write updated record to new-MF
        read next record from MF
        read next record from TF
    else if MF-key < TF-key
        write MF-rec to new-MF
        read next from MF
    end
end-perform