use std::collections::HashMap;

pub type Grammar = HashMap<String, Expression>;

#[derive(Debug, PartialEq, Eq, Clone)]
pub enum Expression {
    Alternative {
        first: Box<Expression>,
        second: Box<Expression>,
        rest: Vec<Expression>,
    },
    Sequence {
        first: Box<Expression>,
        second: Box<Expression>,
        rest: Vec<Expression>,
    },
    Optional(Box<Expression>),
    Repeated(Box<Expression>),
    Factor {
        count: usize,
        primary: Box<Expression>,
    },
    Exception {
        subject: Box<Expression>,
        restriction: Box<Expression>,
    },
    Nonterminal(String),
    Terminal(String),
    Special(String),
    Empty,
}