  We extend all cohomological invariants of similarity classes of quadratic
forms to anti-hermitian forms over a quaternion algebra. This uses the fact
that such invariants can be lifted to Witt invariants, which can be described
as combinations of $\lambda$-operations, and those $\lambda$-operations have
recently been extended to hermitian forms over algebras with involution. In the
article we present a detailed combinatoric description of invariants of
quadratic forms which are partially diagonalized, and show how this
combinatorics extend to anti-hermitian quaternionic forms. The methods
developped here are inteded to be later used for general algebras with
involution.
