MyBool = Inherit {0, 1}
MyBool.
    Fls = MyBool::__new__ 0
    Tru = MyBool::__new__ 1

Expr = Enum {
    .Lit = Int
    .Add = {lhs = Expr; rhs = Expr}
    .Sub = {lhs = Expr; rhs = Expr}
    .Pos = {expr = Expr}
    .Neg = {expr = Expr}
    # .Failure = ...
}
Expr|<: Show|.
    show self = match self:
        Self.Lit(i) -> i.show()
        Self.Add{lhs; rhs} -> "\{lhs}+\{rhs}"
        Self.Sub{lhs; rhs} -> "\{lhs}-\{rhs}"
        Self.Pos{expr;} -> "+\{expr}"
        Self.Neg{expr;} -> "-\{expr}"
Expr.
    eval self = match self:
        Self.Lit(i) -> i
        Self.Add{lhs; rhs} -> lhs.eval() + rhs.eval()
        Self.Sub{lhs; rhs} -> lhs.eval() - rhs.eval()
        Self.Pos{expr;} -> +expr.eval()
        Self.Neg{expr;} -> -expr.eval()

expr = Expr.Add {lhs = Expr.Lit(1); rhs = Expr.Lit(2)}
print! expr # 1 + 2
assert expr.eval() == 3