'Get Token at current input pointer + i ahead where i=1 is next Token.
i<0 indicates tokens in the past.  So -1 is previous token and -2 is
two tokens ago. LT(0) is undefined.  For i>=n, return Token.EOFToken.
Return null for LT(0) and any index that results in an absolute address
that is negative.'

def LT(self, k):

    raise NotImplementedError
