fn read_ahead(tok : Token, mut iterator : Vec<Token>) -> Option<ASTNode> {
        match tok {
            EOF => return None,
            RParen => return None,
            SchemeAtom(s) => return handle_atom(s),
            SchemeString(s) => return Some(StringLiteral(s)),
            LParen => {
                let mut list : Vec<ASTNode> = Vec::new();
                loop {
                    let next = iterator.shift().unwrap();
                    let next_is_rparen = match next { RParen => true, _ => false};
                    if next_is_rparen {
                        return Some(List(list));
                    } else {
                        match read_ahead(next, iterator) {
                            None => return None,
                            Some(tok_) => list.push(tok_),
                        }
                    }
                }
            }
        }
    }