The syntax for -= calls is:
-= is the arithmetic assignment operator for subtraction, it takes $n \geq 2$ input parameters $p_1, \ldots, p_n$ where $p_1$ should be a variable name and the remainder should all be numbers, it sets $p_1 = p_2 - \ldots - p_n$ performed from left to right. For example, $-=(p_1, p_2, p_3, p_4) \Leftrightarrow p_1 = p_2 - p_3 - p_4 = (p_2 - p_3) - p_4$.
Note: It is typically faster to use exprtk for arithmetic assignment operators, plus the syntax is nicer.
Example of -= being used with f++:
:=(int, a=13, b=10)
-=(a, b, 8)
Example of -= being used with n++:
@:=(int, a=13, b=10)
@-=(a, b, 8)