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.