code: public RandomVariable createRandomVariable(RandomVariable randomvariable, boolean isConstant, ArrayList<RandomVariableUniqueVariable> parentVariables, RandomVariableUniqueVariable.OperatorType parentOperatorType) {
  
  		int factoryID = currentFactoryID++;
  
  		listOfAllVariables.add(
  				factoryID,
  				randomvariable
  				);
  
  		return new RandomVariableUniqueVariable(factoryID, isConstant, parentVariables, parentOperatorType);
  	}
docs: Add an object of {@link RandomVariable} to variable list at the index of the current ID
  and rises the current ID to the next one.
  @param randomvariable object of {@link RandomVariable} to add to ArrayList and return as {@link RandomVariableUniqueVariable}
  @param isConstant boolean such that if true the derivative will be set to zero
  @param parentVariables List of {@link RandomVariableUniqueVariable} that are the parents of the new instance
  @param parentOperatorType Operator type
  @return new instance of {@link RandomVariableUniqueVariable}