src/document/doc-utils.js/getDocParts
const getDocParts = (doc) => {
  if (Array.isArray(doc)) {
    return doc;
  }

  /* istanbul ignore next */
  if (doc.type !== "concat" && doc.type !== "fill") {
    throw new Error("Expect doc type to be `concat` or `fill`.");
  }

  return doc.parts;
};