It's not actually Prolog, though I did borrow the :- operator; it's roughly based on the module Parse::RecDescent, but I have taken a few liberties - this is pseudocode.

I'll have a go at putting this into into English:

• We are looking for an expr followed by an end of string
• An expr is a list of comby separated by '|'
• A comby is a list of terms separated by '&'
• A term is either a '(' followed by an expr followed by a ')', or it is a word.

Hope this helps