Function lean_member_idx

crates/kernel/src/ingress.rs:2686–2688

(name: &Name, all: Option<&Vec<Name>>)

      }
    },
    LeanExprData::Mdata(..) => {
      // Unreachable — the while-loop above peeled off every `Mdata` layer.
      unreachable!("Mdata should have been peeled off into mdata_layers");
    },
    LeanExprData::Fvar(name, _) => {
      panic!(