Procedural data type, exercise

MODULE proceduraltype;
//Procedural data type, basic exercise with 2 operations

FROM InOut IMPORT WriteString, WriteLn, Read;
FROM RealInOut IMPORT WriteReal, ReadReal;
VAR
   operation: PROCEDURE(REAL, REAL): REAL;
   nm1, nm2, result : REAL;
   smb : CHAR;
   again : BOOLEAN;

PROCEDURE addition(nmb1, nmb2: REAL): REAL;
VAR
   result : REAL;
BEGIN
     result := nmb1 + nmb2;
     RETURN result
END addition;

PROCEDURE subtraction(nmb1, nmb2: REAL): REAL;
VAR
   result : REAL;
BEGIN
     result := nmb1 - nmb2;
     RETURN result
END subtraction;

BEGIN
     WriteString('Enter first number: ');
     ReadReal(nm1);
     WriteLn;
     WriteString('Enter second number: ');
     ReadReal(nm2);
     WriteLn;
     WriteString('Enter the operation(+ OR -) :');
     again := FALSE;
     REPEAT
           IF again THEN
             WriteString('Enter again, the operation must  be + OR -! : ');
             WriteLn;
           END;
           Read(smb);
           WriteLn;
           again := TRUE;
     UNTIL (smb = '+') OR (smb = '-');
     CASE smb OF
         '+' : operation := addition|
         '-' : operation := subtraction
     END;
     result := operation(nm1,nm2);
     WriteLn;
     WriteString('Result is: ');
     WriteReal(result,0);
     WriteLn;
END proceduraltype.