-= (subtract assign) operator
The right-associative, binary
-= operator decrements it's left-hand operand
by the amount given by its right-hand operand.
variable Integer num = 4; num -= 1; // decrement num by 1 num -= num; // decrement num by 3
-= operator is defined as follows:
lhs = lhs.minus(rhs)
lhs is evaluated only once.
See the language specification for more details.