'-d49.1'' ''''

The checkcompat( ) routine inside conf.c can be tuned to solve many problems (see Appendix D). The default -d49.1 (a.k.a. 49) debugging switch inside it prints the arguments that were passed to it:

checkcompat(to=recipient, from=sender) 

When designing your own checkcompat( ), you should only use the -d49 category to trace it.