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.