// Start gives you the current head point, length is how much data is remaining.
func (*SectionWriter) Unwrap() (r WriterAt, start int64, length int64)

type WriterToAt interface {
  // WriteToAt SHOULD be capable to unpack *SectionWriter objects to accelerate copies.
  // If a copy from an unwrapped *SectionWriter is performed, *SectionWriter also need to be seeked to reflect that.
  // Off is the offset in the reader. If you want pass an offset in the Writer, wrap it in an *SectionWriter first.
  // If the length missmatch between the reader and the writer copy until the smallest one and return EOF.
  // If the copy could not be performed more efficiently than with a simple read write copy loop, then return with errors.ErrUnsupported
  // If the copy was succesfull err == nil and n == length.
  // If length == -1 that means copy until EOF.
  WriteToAt(w Writer, off int64, length int64) (n int64, err error)
}

// Start gives you the current head point, length is how much data is remaining.
func (*SectionReader) Unwrap() (r ReaderAt, start int64, length int64)

type ReaderFromAt interface {
  // ReadFromAt SHOULD be capable to unpack *SectionReader objects to accelerate copies
  // If a copy from an unwrapped *SectionReader is performed, *SectionReader also need to be seeked to reflect that.
  // Off is the offset in the reader. If you want pass an offset in the Reader, wrap it in an *SectionReader first.
  // If the length missmatch between the writer and the reader copy until the smallest one and return EOF.
  // If the copy was succesfull err == nil and n == length.
  // If the copy could not be performed more efficiently than with a simple read write copy loop, then return with errors.ErrUnsupported
  // If length == -1 that means copy until EOF.
  ReadFromAt(w Reader, off int64, length int64) (n int64, err error)
}