src/document/doc-builders.js/concat
function concat(parts) {
  if (process.env.NODE_ENV !== "production") {
    for (const part of parts) {
      assertDoc(part);
    }
  }

  // We cannot do this until we change `printJSXElement` to not
  // access the internals of a document directly.
  // if(parts.length === 1) {
  //   // If it's a single document, no need to concat it.
  //   return parts[0];
  // }
  return { type: "concat", parts };
}