Concatenate the right operand to 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 a concatenation operator.
It also works with numeric values and will add the second to the first.
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.