code: func (rd *RAMDirectory) OpenInput(name string, context IOContext) (in IndexInput, err error) {
  	rd.EnsureOpen()
  	if file, ok := rd.fileMap[name]; ok {
  		return newRAMInputStream(name, file)
  	}
  	return nil, errors.New(name)
  }
docs: // Returns a stream reading an existing file.