code: def merge(self, other):
          
          other = PrefixCell.coerce(other)
          if self.is_equal(other):
              # pick among dependencies
              return self
          elif other.is_entailed_by(self):
              return self
          elif self.is_entailed_by(other):
              self.value = other.value
          elif self.is_contradictory(other):
              raise Contradiction("Cannot merge prefix '%s' with '%s'" % \
                      (self, other))
          else:
              if len(self.value) > len(other.value):
                  self.value = other.value[:]
              # otherwise, return self
          return self
docs: Merges two prefixes