code: public StrBuilder insert(int index, final boolean value) {
          validateIndex(index);
          if (value) {
              ensureCapacity(size + 4);
              System.arraycopy(buffer, index, buffer, index + 4, size - index);
              buffer[index++] = 't';
              buffer[index++] = 'r';
              buffer[index++] = 'u';
              buffer[index] = 'e';
              size += 4;
          } else {
              ensureCapacity(size + 5);
              System.arraycopy(buffer, index, buffer, index + 5, size - index);
              buffer[index++] = 'f';
              buffer[index++] = 'a';
              buffer[index++] = 'l';
              buffer[index++] = 's';
              buffer[index] = 'e';
              size += 5;
          }
          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