function label(label, contents) {
  assertDoc(contents);

  return label ? { type: DOC_TYPE_LABEL, label, contents } : contents;
}
Mark a doc with an arbitrary truthy value. This doesn't affect how the doc is printed, but can be useful for heuristics based on doc introspection.
@param {any} label If falsy, the `contents` doc is returned as is.
@param {Doc} contents