code: def set_unobserved_before(self,tlen,qlen,nt,p):
      
      self._unobservable.set_before(tlen,qlen,nt,p)
docs: Set the unobservable sequence data before this base
  
      :param tlen: target homopolymer length
      :param qlen: query homopolymer length
      :param nt: nucleotide
      :param p: p is the probability of attributing this base to the unobserved error
      :type tlen: int
      :type qlen: int
      :type nt: char
      :type p: float