Runs for the patterns shown in Table 4 take between 5 and 60 minutes on a Silicon Graphics Indigo 2 (150 MHZ IP22 Processor MIPS R4400). The MAP program is available from the authors. Contact GJB by e-mail: gjb@bioch.ox.ac.uk or see the WWW address http://barton.ebi.ac.uk/ for details.