def restrict(a, cron)

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

        cron
      end
Return nil if the cron is "not strict"

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

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