Removes an existing block from the AST.

            `name`
                Block name.

            * Raises a ``ValueError`` exception if `name` hasn't been added.

def remove_block(self, name):
        """ Removes an existing block from the AST.

            `name`
                Block name.

            * Raises a ``ValueError`` exception if `name` hasn't been added.
            """

        if not self._ast or not name in self._block_map:
            raise ValueError(u"Block '{0}' does not exist"
                             .format(common.from_utf8(name)))

        block_idx = self._block_map[name]

        # remove block
        self._ast[2].pop(block_idx)
        del self._block_map[name]