How to specialize seq-concatenate and seq-into-sequence?

[Philipp Stephani]

Hi,

I'm trying to specialize the seq.el functions for a new sequence type.  This is 
mostly straight-forward, except for two functions: seq-concatenate and 
seq-into-sequence.

1. Is the idea that seq-into-sequence should return a "primitive" sequence 
(vector, list, string)?  At least that's my interpretation, but the docstring 
isn't 100% clear.

2. seq-concatenate takes a &rest argument - AIUI it's not possible to 
specialize that?

Thanks,
Philipp