r, w := zeroErrReader{err: er}, errWriter{err: ew}
	n, err := Copy(w, r)
	if n != 0 || err != ew {
		t.Errorf("Copy(zeroErrReader, errWriter) = %d, %v; want 0, writeError", n, err)
	}
}

func TestCopyN(t *testing.T) {
	rb := new(Buffer)
	wb := new(Buffer)
	rb.WriteString("hello, world.")
	CopyN(wb, rb, 5)
	if wb.String() != "hello" {
		t.Errorf("CopyN did not work properly")
	}
}