code: func normalizeHelper(number string,
  	normalizationReplacements map[rune]rune,
  	removeNonMatches bool) string {
  
  	var normalizedNumber = builder.NewBuilder(nil)
  	for _, character := range number {
  		newDigit, ok := normalizationReplacements[unicode.ToUpper(character)]
  		if ok {
  			normalizedNumber.WriteRune(newDigit)
  		} else if !removeNonMatches {
  			normalizedNumber.WriteRune(character)
  		}
  		// If neither of the above are true, we remove this character.
  	}
  	return normalizedNumber.String()
  }
docs: // Normalizes a string of characters representing a phone number by replacing
  // all characters found in the accompanying map with the values therein,
  // and stripping all other characters if removeNonMatches is true.