src/document/doc-builders.js/group
function group(contents, opts = {}) {
  if (process.env.NODE_ENV !== "production") {
    assertDoc(contents);
  }

  return {
    type: "group",
    id: opts.id,
    contents,
    break: Boolean(opts.shouldBreak),
    expandedStates: opts.expandedStates,
  };
}