UnDeclareAllAntiCommutators[] undeclares all
user-defined anti-commutators.
Overview, FCAntiCommutator, CommutatorExplicit, DeclareNonCommutative, DotSimplify.
DeclareNonCommutative[a, b, c, d]
FCAntiCommutator[a, b] = x1;
FCAntiCommutator[c, d] = x2;
DotSimplify[a . b . c . d]b.a.d.c-\text{x2} b.a-\text{x1} d.c+\text{x1} \;\text{x2}
UnDeclareAllAntiCommutators[]
DotSimplify[a . b . c . d]a.b.c.d