[stdout] caught BadInput
error return trace (most recent call last):
  leaf at 243-trace-format.sx:20:16
        if n < 0 { raise error.BadInput; }   // pushes frame 0
                   ^
  mid at 243-trace-format.sx:25:5
        try leaf(n);                          // propagation pushes frame 1
        ^
[stdout] recovered; trace buffer now empty (len 0)