The system can manipulate not only expressions consisting of purely indexed quantities but also expressions containing indexed objects and elementary functions of REDUCE or user defined operators . However there is a natural restriction; the indexed expressions in the arguments of the elementary functions or the operators must not contain free abstract indices i.e., they must be scalar valued expressions. The arguments are regarded and evaluated completely independently:
One can declare differential rules to the user defined operators too (more details about such statements can be found in the following section):