User-defined error
You may want to define your own set of error messages. You can do this with the error command:
Q: error "oops"
E: User-defined error: oops
A more useful formulation might be:
Q: if (year of current date as integer < 2006) then "Still good" else error
"Expired"
E: User-defined error: Expired
The error keyword allows you to set your own, user-defined error codes and implement them in the else clause of an if/then/else statement.