What could explain this behavior?

`test_cobasa' is a binary produced by ocamlc.opt.

,----
| $ echo $OCAMLRUNPARAM
| b
| $ ./test_cobasa -v -f ../test/constraint.cob
| debug1: Parsing '../test/constraint.cob'...
| Fatal error: exception ErrorMsg.Todo
| $ OCAMLRUNPARAM="b" ./test_cobasa -v -f ../test/constraint.cob
| debug1: Parsing '../test/constraint.cob'...
| Fatal error: exception ErrorMsg.Todo
| Raised at file "cobasaParser.mly", line 455, characters 38-51
| Called from file "parsing.ml", line 135, characters 39-75
| Called from file "parsing.ml", line 156, characters 4-28
| Re-raised at file "parsing.ml", line 174, characters 14-17
| Called from file "driver.ml", line 125, characters 12-81
| Called from file "driver.ml", line 165, characters 13-36
| $
`----

Shouldn't the first invocation of test_cobasa give me a stack trace?

-Denis