DeclareNonCommutative[a, b, ...] declares a,b, ... to be non-commutative, i.e., DataType[a,b, ..., NonCommutative] is set to True.
See also: DataType, UnDeclareNonCommutative.
As a side effect of DeclareNonCommutative x is declared to be of data type NonCommutative.
The FeynCalc Book | DataType | DeltaFunction |