Define a variable for simplify

[blevita]

Hi

If i enter some expression to the simplify() function, then it sometimes "assumes" some values for some variables.

Is it possible to define the value of some variables instead of replacing them directly?

As an example:

simplify(log_a^5(a^8))

It will assume a to be near to zero.
Is there a way to define a instead like that:

simplify(log_a^5(a^8) | a=2)

Or basically also for other calculations?

x+7-9 | x=3

TI-Nspire has such a functionality.
It would be great to here what is possible with the prime.
I have also attached a picture.

Thanks