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.