code: func TopLevelContext(ctx IndexReaderContext) IndexReaderContext {
  	for ctx.Parent() != nil {
  		ctx = ctx.Parent()
  	}
  	return ctx
  }
docs: // index/ReaderUtil.java
  /* Common util methods for dealing wiht IndexReaders and IndexReaderContexts. */
  /*
  Walks up the reader tree and return the given context's top level
  reader context, or in other words the reader tree's root context.
  */