code: public static String replaceIfFirstChar(String s, char c, String replacement) {
          return s.charAt(0) == c ? replacement + s.substring(1) : s;
      }
docs: Replaces the char c in the string S, if it's the first character in the string.
  @param s The string to search
  @param c The character to replace
  @param replacement The string to replace the character in S
  @return The string with the character replaced (or the original string if the char is not found)