code: func foldFunc(s []byte) func(s, t []byte) bool {
  	nonLetter := false
  	special := false // special letter
  	for _, b := range s {
  		if b >= utf8.RuneSelf {
  			return bytes.EqualFold
  		}
  		upper := b & caseMask
  		if upper < 'A' || upper > 'Z' {
  			nonLetter = true
  		} else if upper == 'K' || upper == 'S' {
  			// See above for why these letters are special.
  			special = true
  		}
  	}
  	if special {
  		return equalFoldRight
  	}
  	if nonLetter {
  		return asciiEqualFold
  	}
  	return simpleLetterEqualFold
  }
docs: // foldFunc returns one of four different case folding equivalence
  // functions, from most general (and slow) to fastest:
  //
  // 1) bytes.EqualFold, if the key s contains any non-ASCII UTF-8
  // 2) equalFoldRight, if s contains special folding ASCII ('k', 'K', 's', 'S')
  // 3) asciiEqualFold, no special, but includes non-letters (including _)
  // 4) simpleLetterEqualFold, no specials, no non-letters.
  //
  // The letters S and K are special because they map to 3 runes, not just 2:
  //  * S maps to s and to U+017F 'ſ' Latin small letter long s
  //  * k maps to K and to U+212A 'K' Kelvin sign
  // See http://play.golang.org/p/tTxjOc0OGo
  //
  // The returned function is specialized for matching against s and
  // should only be given s. It's not curried for performance reasons.