The most important objects are the tensors. The identifiers of tensors must have been defined by the command TENSOR before they could be applied:
The identifiers D and LD cannot be introduced as tensors because they denote the operators of covariant and Lie derivatives. If a tensor identifier should be reintroduced to the system as an object of another type (e.g. symbol) then it should be cleared first by the command CLEAR
Warning: a tensor identifier carries all the information about the tensors with this name but of different types, values, and symmetry properties. Applying the command CLEAR above will destroy every piece of information about them.