Other forms of the IF-THEN statement
Note that the expression in the first IF statement is a complex one:

IF 0 LE Score LT 65 THEN Grade='F';

It can be rewritten any of the following ways (with identical results):

IF 0 <= Score < 65 THEN Grade='F';
IF (0 LE Score) and (Score LT 65) THEN Grade='F';
IF (Score GE 0)  &  (Score LT 65) THEN Grade='F';