I'm studying for a SAS certification exam, and I came across an unexplained behavior. Note the data step below:

`data D;`  

`A+1;`  

`A+1;`  

`A+1;`  

`run;`

Question 1: Why this step does not result in error?  

Question 2: Why a variable A is created, and its value is 3 and not missing?  

Question 3: Why when I change + for - , it results in error?

I have searched about it and i couldn't find nothing, even in SAS documentation