Returns the result of concatenating tailvec to the implicit
        parameter

def concat(self, tailvec):
        '''Returns the result of concatenating tailvec to the implicit
        parameter'''
        newvec = ImmutableVector()
        vallist = [(i + self._length, tailvec[i]) \
                for i in range(0, tailvec._length)]
        newvec.tree = self.tree.multi_assoc(vallist)
        newvec._length = self._length + tailvec._length
        return newvec