function foo(cond) {
  let a = {};
  let b = {};
  let c = {};
  while (cond) {
    let z = a;
    a = b;
    b = c;
    c = z;
    a |> mutate(%, b);
  }
  a;
  b;
  c;
  return a;
}
function mutate(x, y) {}