docs: def grp_start_len(a):
      """Given a sorted 1D input array `a`, e.g., [0 0, 1, 2, 3, 4, 4, 4], this
      routine returns the indices where the blocks of equal integers start and
      how long the blocks are.
      """
      # https://stackoverflow.com/a/50394587/353337
      m = numpy.concatenate([[True], a[:-1] != a[1:], [True]])
      idx = numpy.flatnonzero(m)
      return idx[:-1], numpy.diff(idx)