Put the inequality of delete_singleton_ne in the same direction as other _ne lemmas.

4 jobs for master in 11 minutes and 5 seconds
Status Job ID Name Coverage
  Test
passed #3225
fp-timing
stdpp-coq8.5.3

00:02:02

passed #3224
fp-timing
stdpp-coq8.6

00:01:59

passed #3223
fp-timing
stdpp-coq8.6.1

00:09:04

passed #3222
fp-timing
stdpp-coq8.7

00:07:03