λ-calculus Parser (using LALRPOP)

```rust use lalrpop_lambda::lambda::ExpressionParser; let parser = ExpressionParser::new();

// Parse a single free variable. let x = parser.parse("x");

// Parse the identity function. let id = parser.parse(r"\x.x");

// f ∘ g let compose = parser.parse(r"\f.\g.\x.(f (g x))"));

// Print the free variable in this expression. let unboundy = parser.parse(r"\x.x y"); println!("{}", unboundy.free_variables());

// No need for parsing strings at all. let id = λ!{x.x}; let one = λ!{f.λ!{x.γ!(f, x)}};

// Identity application. let id = λ!{x.x}; println!("(id one): {} -> {}", app!({&id}, {&one}), app!({&id}, {&one}).normalize(false));

// Make the Y combinator. let ω = parser.parse(r"λx.(x x)"); let Ω = parser.parse(r"(λx.(x x)) (λx.(x x))"); let W = parser.parse(r"λf.λx. f x x"); let Y = parser.parse(r"λf.(λx.f (x x)) (λx.f (x x))"); ```