error: unhandled error reached main: error.A
error return trace (most recent call last):
  fa at 247-failable-or-chain-propagate.sx:14:17
        if n == 0 { raise error.A; }
                    ^
  main at 247-failable-or-chain-propagate.sx:19:10
        v := try fa(0) or try fa(0) or try fa(0);   // all fail → propagate to main
             ^
  fa at 247-failable-or-chain-propagate.sx:14:17
        if n == 0 { raise error.A; }
                    ^
  main at 247-failable-or-chain-propagate.sx:19:10
        v := try fa(0) or try fa(0) or try fa(0);   // all fail → propagate to main
             ^
  fa at 247-failable-or-chain-propagate.sx:14:17
        if n == 0 { raise error.A; }
                    ^
  main at 247-failable-or-chain-propagate.sx:19:10
        v := try fa(0) or try fa(0) or try fa(0);   // all fail → propagate to main
             ^