docs: def dna_to_re(seq):
      """
      Return a compiled regular expression that will match anything described by 
      the input sequence.  For example, a sequence that contains a 'N' matched 
      any base at that position.
      """
  
      seq = seq.replace('K', '[GT]')
      seq = seq.replace('M', '[AC]')
      seq = seq.replace('R', '[AG]')
      seq = seq.replace('Y', '[CT]')
      seq = seq.replace('S', '[CG]')
      seq = seq.replace('W', '[AT]')
      seq = seq.replace('B', '[CGT]')
      seq = seq.replace('V', '[ACG]')
      seq = seq.replace('H', '[ACT]')
      seq = seq.replace('D', '[AGT]')
      seq = seq.replace('X', '[GATC]')
      seq = seq.replace('N', '[GATC]')
  
      return re.compile(seq)