code: private void addElementToNextFreeSlot(@Nonnull final ExpressionTreeElement element) {
      if (element == null) {
        throw new PreprocessorException("[Expression]Element is null", this.sourceString, this.includeStack, null);
      }
  
      if (childElements.length == 0) {
        throw new PreprocessorException("[Expression]Unexpected element, may be unknown function [" + savedItem.toString() + ']', this.sourceString, this.includeStack, null);
      } else if (isFull()) {
        throw new PreprocessorException("[Expression]There is not any possibility to add new argument [" + savedItem.toString() + ']', this.sourceString, this.includeStack, null);
      } else {
        childElements[nextChildSlot++] = element;
      }
      element.parentTreeElement = this;
    }
docs: Add an expression element into the next free child slot
  
  @param element an element to be added, must not be null