Specification of sequence length in number of elements

[treiher]

The length of an sequence is often defined by a length field. But some protocols like USB and DNS only specify the number of elements of an sequence. In such a case the length of the sequence field can only be determined statically if the length of the elements is known and homogeneous. Otherwise the length has to be determined dynamically at runtime (requires #32).

An aspect has to be added to be able to represent these protocols.