code: func simpleLetterEqualFold(s, t []byte) bool {
  	if len(s) != len(t) {
  		return false
  	}
  	for i, b := range s {
  		if b&caseMask != t[i]&caseMask {
  			return false
  		}
  	}
  	return true
  }
docs: // simpleLetterEqualFold is a specialization of bytes.EqualFold for
  // use when s is all ASCII letters (no underscores, etc) and also
  // doesn't contain 'k', 'K', 's', or 'S'.
  // See comments on foldFunc.