code: def is_downstream_of(self, other):
          
          if self.chrom != other.chrom: return None
          if getattr(other, "strand", None) == "-":
              # other feature is on - strand, so this must have higher start
              return self.end <= other.start
          return self.start >= other.end
docs: return a boolean indicating whether this feature is downstream of
          `other` taking the strand of other into account