error: GraphQL union expects trait method to accept `&self` only and, optionally, `&Context`
 --> fail/union/trait_wrong_method_input_args.rs:5:10
  |
5 |     fn a(&self, ctx: &(), rand: u8) -> Option<&Human>;
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: https://spec.graphql.org/June2018/#sec-Unions