Program stack overflow defining infix operator



Hello everybody,

(%i1) infix("imp", 50, 50, clause, clause, clause);

(%o1)                                 imp
(%i2) "imp"(a, b):= not(a) or b;

(%o2)                      (a imp b) := (not a) or b
(%i3) expand(a imp b);
(%o3)                            (not a) or b
(%i4) expand(a and b imp c);

*** - Program stack overflow. RESET

[/build/buildd/clisp-2.44.1/src/eval.d:527] reset() found no driver 
frame (sp=0xbfff8fd0-0xbfff2ad0)
Exiting on signal 6

Process maxima aborted

What I'm doing wrong?
Thanks for any help, regards
Stephan