code: public CALC use(Class<? extends Extension> operationClass) {
          if (useExtensions == null)
              useExtensions = new UseExtension();
  
          if (Operator.class.isAssignableFrom(operationClass))
              useExtensions.registerOperator(operationClass.asSubclass(Operator.class));
          if (Function.class.isAssignableFrom(operationClass))
              useExtensions.registerFunction(operationClass.asSubclass(Function.class));
  
          return getThis();
      }
docs: Use custom {@link Operator} or {@link Function} inside scope of this calculation, which can be used during parsing expression.
  With using custom extension it's possible to override existing extension from global scope.
  Because during calculation API first scan scoped (locally used) extensions and after that search in global extension from {@link CacheExtension}
  
  @param operationClass
  @return