Require amounts to be nonnegative in semantics

3 jobs for master in 7 minutes and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #100832
au
coq:8.8

00:03:45

passed #100833
au
coq:8.9

00:03:46

passed #100834
au
coq:dev

00:03:41