docs: def gapmask(simseqs, origseqs):
      """
      :param sims: list of (header, sequence) tuples of simulated sequences [no gaps]
      :param aln: list of (header, sequence) tuples of original sequences
      :return:
      """
      import numpy as np
      simdict = dict(simseqs)
      origdict = dict(origseqs)
      for k in origdict:
          origseq = np.array(list(origdict[k]))
          gap_pos = np.where(origseq=='-')
          simseq = np.array(list(simdict[k]))
          simseq[gap_pos] = '-'
          simdict[k] = ''.join(simseq)
      return list(simdict.items())