What happens if I execute an SKAction run sequence on a node that is already executing another sequence?

I have an SKSpriteNode with an SKAction being run on it:

theGem!.run(premAction, completion: {theGem!.run(repeatAction)})

Can't seem to find out the proper steps to run another action, such as:

theGem.run(endsequence, completion: {theGem.removeAllActions(); theGem.run(stopAction)})

Should I stop the previous action first? Is there a way to turn the repeat part off so that the first SKAction ends smoothly?

What happens if I execute an SKAction run sequence on a node that is already executing another sequence?
 
 
Q