HB.saturare key hangs

[damien-pous]

I have a case where HB.saturate on a given key hangs: https://github.com/damien-pous/partial-orders/blob/eb3b26da87156f8ecab28169e7b190365403c194/theories/lattice.v#L15

It's rather hard to minimise; the strange thing is that HB.saturate on a rather similar key (monotone functions rather than extensive ones) just works: https://github.com/damien-pous/partial-orders/blob/eb3b26da87156f8ecab28169e7b190365403c194/theories/lattice.v#L16

Cc-ing @CohenCyril who encouraged me to submit this issue..