Subtracts the right operand from the left operand and assign the result to the left operand.
This is functionally equivalent to the expression:
anOperand1 = anOperand1 - anOperand2;
Although this is classified as an assignment operator it is really a compound of an assignment and an additive operator.
The associativity is right to left.
Refer to the operator precedence topic for details of execution order.
The new value of anOperand1 is returned as a result of the expression.
The operand to the left of the operator must be an LValue. That is, it should be able to take an assignment and store the value.