FEATURE REQUEST: New validation rule similar to declined_if

[kitihounel]

Hello.
I think it will be useful to add a new validation rule similar to declined_if.
To quote the doc of declined_if: The field under validation must be "no", "off", 0, or false if another field under validation is equal to a specified value.
The new validation rule should forbid a value (or certain values) for the field under validation if another field has a value (or certain values).
The syntax could be:
forbidden_if:<forbidden value>,<other field>,<other field value>

Use case
Suppose that we have a page that does arithmetic operations (add, minus, mul and div) and accepts the two operands (a and b) and the operator (op) via POST. For the division operation, the second operand should not be null. So the validation rules should be:

$rules = [
    'a' => 'required|numeric',
    'b' => 'required|numeric|forbidden_if:0,op,div',
    'op' => 'required|string|in:add,minus,mul,div'
];

I looked in the validation rules available, and there no rule that does that.