The goal of the parser is to accept as many correct sentences as possible, while also accepting wrong sentences. If I take your example of "last $1", this means that you can just put all the words that could replace "last" in the list: "A|B|C|D $1". This way, when the user uses the correct word according to $1, the sentence will be parsed. The fact that the user is allowed to use C instead of B with $1 being a day is not a problem at all.