as: Fail

 
 7.34 '.fail EXPRESSION'
 =======================
 
 Generates an error or a warning.  If the value of the EXPRESSION is 500
 or more, 'as' will print a warning message.  If the value is less than
 500, 'as' will print an error message.  The message will include the
 value of EXPRESSION.  This can occasionally be useful inside complex
 nested macros or conditional assembly.