bagIterUnion = xyi => {
  /** @type {[T,bigint][]} */
  const result = [];
  for (const [m, xc, yc] of xyi) {
    result.push([m, xc + yc]);
  }
  return result;
}
@template T
@param {[T,bigint,bigint][]} xyi
@returns {[T,bigint][]}