public static ReferenceUnit addReferenceUnit(String name, String typeName, String systemName) {
            
            ReferenceUnit createdUnit = null;

            if(!listUnits.containsKey(name) && listTypes.containsKey(typeName) && listSystems.containsKey(systemName)) {
                Type type = listTypes.get(typeName);
                System system = listSystems.get(systemName);

                createdUnit = new ReferenceUnit(name, type, system);
                type.setReferenceUnit(createdUnit);

                type.getUnits().put(name, createdUnit);
                system.getUnits().put(name, createdUnit);
                
                listUnits.put(name, createdUnit);
            }
            
            return createdUnit;
	}