[AS3] #1050 cannot assign a non-reference value

Actionscript 3.0 do not accept some type of the Variable.
if you use the illegal method to name the Variable,
it will appear the error.

wrong code:

"coin_atStage"+j.y -=10;

the code below is not good but the error disappear when i change it:

coin_atStage.y -=10;