code: public static StringBuffer insert(StringBuffer target, int offset16, int char32) {
          String str = valueOf(char32);
          if (offset16 != target.length() && bounds(target, offset16) == TRAIL_SURROGATE_BOUNDARY) {
              offset16++;
          }
          target.insert(offset16, str);
          return target;
      }
docs: Inserts char32 codepoint into target at the argument offset16. If the offset16 is in the
  middle of a supplementary codepoint, char32 will be inserted after the supplementary
  codepoint. The length of target increases by one if codepoint is non-supplementary, 2
  otherwise.
  <p>
  The overall effect is exactly as if the argument were converted to a string by the method
  valueOf(char) and the characters in that string were then inserted into target at the
  position indicated by offset16.
  </p>
  <p>
  The offset argument must be greater than or equal to 0, and less than or equal to the length
  of source.
  
  @param target String buffer to insert to
  @param offset16 Offset which char32 will be inserted in
  @param char32 Codepoint to be inserted
  @return a reference to target
  @exception IndexOutOfBoundsException Thrown if offset16 is invalid.