error:
       … from call site
         at /pwd/in.nix:4:1:
            3| in
            4| x "x"
             | ^
            5|

       … while calling 'x'
         at /pwd/in.nix:2:7:
            1| let
            2|   x = arg: assert arg == "y"; 123;
             |       ^
            3| in

       error: assertion failed
       at /pwd/in.nix:2:12:
            1| let
            2|   x = arg: assert arg == "y"; 123;
             |            ^
            3| in
