if b.Flush(); b.err != nil {
			return 0, b.err
		}
		n = b.Available()
		if n < utf8.UTFMax {
			// Can only happen if buffer is silly small.
			return b.WriteString(string(r))
		}
	}
	size = utf8.EncodeRune(b.buf[b.n:], r)
	b.n += size
	return size, nil
}

// WriteString writes a string.
// It returns the number of bytes written.
// If the count is less than len(s), it also returns an error explaining

		// The distribution of the last byte is more uniform compared to the
		// first byte which has a 78% chance of being [240, 243, 244].
		var b [utf8.UTFMax]byte
		n := utf8.EncodeRune(b[:], r)
		last := n - 1
		i := last
		fails := 0
		for i < len(s) {
			if s[i] != b[last] {
				o := IndexByte(s[i+1:], b[last])
				if o < 0 {
					return -1
				}
				i += o + 1
			}

			}
		}
		buf[n-1] = '\x00'
	}
}

func bmIndexRune(index func([]byte, rune) int) func(b *testing.B, n int) {
	return func(b *testing.B, n int) {
		buf := bmbuf[0:n]
		utf8.EncodeRune(buf[n-3:], '世')
		for i := 0; i < b.N; i++ {
			j := index(buf, '世')
			if j != n-3 {
				b.Fatal("bad index", j)
			}
		}
		buf[n-3] = '\x00'
		buf[n-2] = '\x00'
		buf[n-1] = '\x00'
	}
}