code: def assoc(self, index, value):
          '''Return a new vector with value associated at index. The implicit
          parameter is not modified.'''
          newvec = ImmutableVector()
          newvec.tree = self.tree.assoc(index, value)
          if index >= self._length:
              newvec._length = index+1
          else:
              newvec._length = self._length
          return newvef assoc(self, index, value):
          '''Return a new vector with value associated at index. The implicit
          parameter is not modified.'''
          newvec = ImmutableVector()
          newvec.tree = self.tree.assoc(index, value)
          if index >= self._length:
              newvec._length = index+1
          else:
              newvec._length = self._length
          return newvec
docs: Return a new vector with value associated at index. The implicit
          parameter is not modified.