 notation.
    When the expression is known, it is assigned to the ``Forward``
    variable using the ``'<<'`` operator.

    Note: take care when assigning to ``Forward`` not to overlook
    precedence of operators.

    Specifically, ``'|'`` has a lower precedence than ``'<<'``, so that::

        fwd_expr << a | b | c

    will actually be evaluated as::

        (fwd_expr << a) | b | c

    thereby leaving b and c out as parseable alternatives.  It is recommended that you
    explicitly group the values inserted into the ``Forward``::

        fwd_expr << (a | b | c)

    Converting to use the ``'<<='`` operator instead will avoid this problem.

    See :class:`ParseResults.pprint` for an example of a recursive
    parser created using ``Forward``.
    N)