code: public StrBuilder insert(final int index, final char value) {
          validateIndex(index);
          ensureCapacity(size + 1);
          System.arraycopy(buffer, index, buffer, index + 1, size - index);
          buffer[index] = value;
          size++;
          return this;
      }
docs: Inserts the value into this builder.
  
  @param index  the index to add at, must be valid
  @param value  the value to insert
  @return this, to enable chaining
  @throws IndexOutOfBoundsException if the index is invalid