[2018-11 LWG Motion 25] P0896R4 -- Clause 17

[zygoloid]

P0896R4
Edits to Clause 17. To be applied to motions-2018-11-lwg-25 branch.

[zygoloid]

Source:

https://github.com/ericniebler/stl2/blob/3b4b81a961b17528d38589c1dfb0c41b4228c6a8/ext/merge2std2/concepts.tex

[jensmaurer]

Done and pushed.