You can increase the value of a variable a by 4, for example, with
| a:=a+4 | 
If beforehand a were equal to 4, it would now be equal to 8. A shorthand way of doing this is with the += operator;
| a+=4 | 
will also increase the value of a by 4.
Similar shorthands exist for subtraction, multiplication and division. If a is equal to 8 and you enter
| a-=2 | 
then a will be equal to 6. If you follow this with
| a*=3 | 
then a will be equal to 18, and finally
| a/=9 | 
will end with a equal to 2.