'Move a token from the beginning of ``remaining_text`` to the
end of ``stack``.
:type stack: list(str and Tree)
:param stack: A list of strings and Trees, encoding
the structure of the text that has been parsed so far.
:type remaining_text: list(str)
:param remaining_text: The portion of the text that is not yet
covered by ``stack``.
:rtype: None'

def _shift(self, stack, remaining_text):

    stack.append(remaining_text[0])
    remaining_text.remove(remaining_text[0])
    if self._trace:
        self._trace_shift(stack, remaining_text)
