Again, this is an issue of sticking to the concept* of concatenation while the types you have at hand aren't a good fit for it due to ambiguity (contrary to a linked list, for example).
Either you have a growable sequence or an immutable one, you need to iterate the items you want to grow it with or the items of both containers respectfully to make a new one. You can have some sugar over it, but with sequences it all boils down to iteration (or moving chunks of memory), it's the nature of the type. What you want in your test example is some form of `extend` which grows a mutable container (your `var seq`) with the elements of an iterator (can we have this today using some iterator concept?), or `extendFromArray`, as a specialization of the same operation. *in a common sense
