class Un

expr::Node tree = expr::Un("-", expr::Field(["balance"]));
match (tree) {
    expr::Un => { expr::Un u = tree; console.writeln("unary ${u.op}"); }
    else => { console.writeln("not a Un"); }
}

unary -

new(string o, Node inner)

expr::Un u = expr::Un("!", expr::Field(["active"]));
console.writeln(u.op);
match (u.e) {
    expr::Field => { console.writeln("operand is a field"); }
    else => { console.writeln("operand is something else"); }
}

!
operand is a field

Node e

string op