ASSERT

 The ASSERT procedure signals a run-time error if  the  value  of
 its parameter is FALSE.


 Syntax:

    ASSERT(expression [[, string]])


 The 'expression' is a Boolean expression that is normally  true.
 If  ASSERT  evaluates  the  expression  as  false,  it signals a
 run-time error, indicating that the assertion failed.

 The optional string parameter is output as  part  of  the  error
 message.