Consider the alphabet $Σ = \{1, +, =\}$ and the following language, $PLUS = \{ 1^m + 1^n = 1^{m+n} \mid m, n ∈ ℕ \}$. Prove with Myhill-Nerode that PLUS is not a regular language.
I know how I should use Myhill-Nerode here and how I should show that any two strings in the infinite set $S$ are distinguishable, but I am stuck at defining my infinite set S for this language. I tried $S = \{1^n \mid \text{$n$ is a natural number} \}$ but I don't think it works.