code: public JBBPDslBuilder Bits(final String name, final String bitLenExpression) {
      final Item item = new Item(BinType.BIT, name, this.byteOrder);
      item.bitLenExpression = assertExpressionChars(bitLenExpression);
      this.addItem(item);
      return this;
    }
docs: Add named bit field which length calculated by expression.
  
  @param name             name of the field, if null then anonymous one
  @param bitLenExpression expression to calculate number of bits, must not be null
  @return the builder instance, must not be null