code: public void registerContentOperator(String operatorString, ContentOperator operator) {
  		if (operators.containsKey(operatorString)) {
  			throw new IllegalArgumentException("Operator " + operatorString + " already registered");
  		}
  		operators.put(operatorString, operator);
  	}
docs: Registers a content operator that will be called when the specified operator string is encountered during content processing.
  Each operator may be registered only once (it is not legal to have multiple operators with the same operatorString)
  @param operatorString the operator id
  @param operator the operator that will receive notification when the operator is encountered
  @since 2.1.8