'Get tree node at current input pointer + i ahead where i=1 is next node.
i<0 indicates nodes in the past.  So LT(-1) is previous node, but
implementations are not required to provide results for k < -1.
LT(0) is undefined.  For i>=n, return null.
Return null for LT(0) and any index that results in an absolute address
that is negative.
This is analogus to the LT() method of the TokenStream, but this
returns a tree node instead of a token.  Makes code gen identical
for both parser and tree grammars. :)'

def LT(self, k):

    raise NotImplementedError
