code: func NewWrapper(rwa ReadWriterAt, L, O, N int64) *Wrapper {
  	return &Wrapper{
  		L:   L,
  		O:   O,
  		N:   N,
  		rwa: rwa,
  	}
  }
docs: // NewWrapper creates a Wrapper based on ReadWriterAt rwa.
  // L is the current length, O is the current offset, and N is offset at which we "wrap".