pub fn contains_symbol(&self, sym: &Symbol) -> bool {
        let mut found = false;
        self.traverse(&mut |ref mut e| {
            if let ExprKind::Ident(ref s) = e.kind {
                if *sym == *s {
                    found = true;
                }
            }
        });
        found
    }
Returns `true` if this expression contains the symbol `sym` in an `Ident`.
weld/src/ast/ast.rs