User-syntax for disabling positivity checking

[coqbot]

Note: the issue was created automatically with bugzilla2github tool

Original bug ID: BZ#5695
From: @JasonGross
Reported version: 8.7+alpha
CC: @herbelin, @ppedrot

[coqbot]

Comment author: @JasonGross

#79 was merged a year ago, right before the v8.6
deadline. Coq v8.7 still has no syntax for this. Can we get some syntax for
8.8? (And maybe a plugin with syntax for 8.7?)

[coqbot]

Comment author: @ppedrot

This depends on the attribute CEP, which is still to be found...

[ppedrot]

Fixed by #10291