class Fugit::Nat::SlotGroup

It happens here because it's nat being strict, not cron.

from "every 17 hours".
cron string, but makes not much sense when derived via `.parse_nat`
For example, "0 0/17 * * *" (gh-86) is a perfectly valid

Return nil if the cron is "not strict"

def restrict(a, cron)
  if m = ((a[1] && a[1][0]) || '').match(/^(\d+|\*)\/(\d+)$/)
     sla = m[1].to_i
    return nil unless [ 1, 2, 3, 4, 5, 6, 8, 12 ].include?(sla)
  end
  cron
end