error:
       … while evaluating the attribute 'body'
         at /pwd/lang/eval-fail-assert.nix:7:3:
            6|
            7|   body = x "x";
             |   ^
            8| }

       … from call site
         at /pwd/lang/eval-fail-assert.nix:7:10:
            6|
            7|   body = x "x";
             |          ^
            8| }

       … while calling 'x'
         at /pwd/lang/eval-fail-assert.nix:3:5:
            2|   x =
            3|     arg:
             |     ^
            4|     assert arg == "y";

       … while evaluating the condition of the assertion '(arg == "y")'
         at /pwd/lang/eval-fail-assert.nix:4:5:
            3|     arg:
            4|     assert arg == "y";
             |     ^
            5|     123;

       error: string '"x"' is not equal to string '"y"'