Mutating in-place replacement of a substring of a String

String+replaceOccurence.swift

// known-good Swift3

extension String {
  /// Performs in-place replacement of first occurence of a substring
  mutating func replaceFirstOccurence(of query:String, with replacement:String) {
    guard let firstRange = self.range(of: query)
      else { return }
    self.replaceSubrange(firstRange, with: replacement)
  }
}