COM's question

I appreciate if somebody tell me how to do the following:



In COM module, immediate order entry we do a credit memo and need to protect
that the user not to use the sequences 010 thru 050 because corresponds to
an invoice.



I try to solve in navigation screen for an user, but only present an initial
screen default. The user can reach the all the sequences by typing one of
then without a restriction.



Another solution I think is with an user exit program that validate the
quantity of a credit memo to be negative in order to be accepted.



Regards!