?s=sabc

A cryptic puzzle!

Let's see... if ?s=sabc, that means that the unknown character ? is followed by the letter s, and the result is equal to the string sabc.

Hmm... could it be that ? is the letter a?

If so, then as would equal sabc, which is true!

So, I'm going to take a guess that ? is indeed the letter a. Am I correct?