docs: def mask_seq(self,seq):
          """
          m.mask_seq(seq) -- Return a copy of input sequence in which any regions matching m are replaced with strings of N's
          """
          masked = ''
          matches, endpoints, scores = self.scan(seq)
          cursor = 0
          for start, stop in endpoints:
              masked = masked + seq[cursor:start] + 'N'*self.width
              cursor = stop+1
          masked = masked + seq[cursor:]
          return masked