Add tests and commentary about `∈` in non-loop contexts

Closes #17.

README.md

@@ -514,10 +514,9 @@ The keyword `in` is used consistently instead of `∈` and `=` in `for` loops. E
 +for i in 1:2
 ```
 
-#### Potential changes
- - Since `∉` exists the formatted code may become asymmetric, when both `∈` and `∉` are
-   used, since `∉` is left alone and not replaced by e.g. `!(...)`. See discussion in issue
-   [#17](https://github.com/fredrikekre/Runic.jl/issues/17).
+Note that `∈` not replaced when used as an operator outside of loop contexts in
+order to be symmetric with `∉` which doesn't have a direct ASCII equivalent.
+See [#17](https://github.com/fredrikekre/Runic.jl/issues/17) for more details.
 
 ### Braces around right hand side of `where`
 

test/runtests.jl

@@ -467,6 +467,11 @@ end
                 "$(l)i for i in I, j in J if i < j$(r)"
         end
     end
+    # ∈ is still allowed when used as an operator outside of loop contexts in order to keep
+    # symmetry with ∉ which doesn't have a direct ascii equivalent.
+    # See https://github.com/fredrikekre/Runic.jl/issues/17
+    @test format_string("a ∈ A") == "a ∈ A"
+    @test format_string("a ∉ A") == "a ∉ A"
 end
 
 @testset "braces around where rhs" begin