src/document/doc-builders.js/group
function group(contents, opts) {
  opts = opts || {};

  if (process.env.NODE_ENV !== "production") {
    assertDoc(contents);
  }

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