code: def subtract(self, es):
      
      workingSet = [self]
      for e in es:
        newWorkingSet = []
        for w in workingSet:
          newWorkingSet += w.__singleIntersect(e)
        workingSet = newWorkingSet
      return workingSet
docs: Subtract the BED elements in es from self.
  
      :param es: a list of BED elements (or anything with chrom, start, end)
      :return: a list of BED elements which represent what is left of
               self after the subtraction. This might be an empty list.