function ifBreak(breakContents, flatContents = "", opts = {}) {
  assertDoc(breakContents);
  if (flatContents !== "") {
    assertDoc(flatContents);
  }

  return {
    type: DOC_TYPE_IF_BREAK,
    breakContents,
    flatContents,
    groupId: opts.groupId,
  };
}
@param {Doc} breakContents
@param {Doc} [flatContents]
@param {object} [opts] - TBD ???
@returns Doc