The docs on ReaderFrom are very light:

http://golang.org/pkg/io/#ReaderFrom

`
type ReaderFrom interface {
    ReadFrom(r Reader) (n int64, err error)
}
ReaderFrom is the interface that wraps the ReadFrom method.
`

But it doesn't say what the method is expected to do.

The *bytes.Buffer ReadFrom provides some clues:

http://golang.org/pkg/bytes/#Buffer.ReadFrom

... but is that the promise from a *bytes.Buffer, or of all ReadersFrom?