code: func asciiEqualFold(s, t []byte) bool {
  	if len(s) != len(t) {
  		return false
  	}
  	for i, sb := range s {
  		tb := t[i]
  		if sb == tb {
  			continue
  		}
  		if ('a' <= sb && sb <= 'z') || ('A' <= sb && sb <= 'Z') {
  			if sb&caseMask != tb&caseMask {
  				return false
  			}
  		} else {
  			return false
  		}
  	}
  	return true
  }
docs: // asciiEqualFold is a specialization of bytes.EqualFold for use when
  // s is all ASCII (but may contain non-letters) and contains no
  // special-folding letters.
  // See comments on foldFunc.