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This creature enters with four +1/+0 counters on it.
This creature can't be blocked by artifact creatures.
At end of combat, if this creature attacked or blocked this combat, remove a +1/+0 counter from it.

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: Put up to X +1/+0 counters on this creature. This ability can't cause the total number of +1/+0 counters on this creature to be greater than four. Activate only during your upkeep.
If Clockwork Steed has four or fewer +1/+0 counters on it when its last ability resolves, it can wind up with a maximum of four such counters on it. If it has four or more +1/+0 counters on it, the ability will have no effect.