decrement Commanddecrement <target> [by <number>]
The decrement command subtracts from an existing variable, property, or attribute. It defaults to subtracting the value 1, but this can be changed using the by modifier. If the target variable is null, then it is assumed to be 0, and then decremented by the specified amount. The decrement command is the opposite of the increment command command.
If you target a string variable, then decrement uses += to add the string to the end of the target variable.
set counter to 5
decrement counter by 2 -- counter is now 3
decrement newVariable -- newVariable is defaulted to zero, then decremented to -1