NOT
The NOT operator
performs a logical negation to invert a boolean expression.
Syntax
NOT bool-expr 
 - bool-expr is a boolean expression.
 
Usage
The NOT operator is typically used to invert the value of a boolean
expression.
If the operand is NULL, the negation expression evaluates to
NULL.
Example
MAIN
  IF NOT ( 256 == 257 ) THEN
     DISPLAY "This line should display"
  END IF
END MAIN