error:
       … while calling the 'toString' builtin
         at /pwd/in.nix:1:1:
            1| toString { __toString = 1; }
             | ^
            2|

       … while converting a set to string

       error: attempt to call something which is not a function but an integer: 1
       at /pwd/in.nix:1:12:
            1| toString { __toString = 1; }
             |            ^
            2|
