λ-calculus Parser (using LALRPOP)
use lalrpop_lambda::lambda::ExpressionParser;
let parser = ExpressionParser::new();
let x = parser.parse("x");
let id = parser.parse(r"\x.x");
let compose = parser.parse(r"\x.\y.x y"));
let unbound_y = parser.parse(r"\x.x y");
println!("{}", unbound_y.free_variables());
let id = λ!{x.x};
let one = λ!{f.λ!{x.γ!(f, x)}};
println!("(normalize (id one)): {}",
app!(id, one).normalize(false));
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))");