a^n (advanced) {it}/{rec} solution

MODULE z;
FROM InOut IMPORT WriteString, WriteLn, WriteInt, ReadInt, Read;
FROM RealInOut IMPORT ReadReal, WriteReal;

VAR
   a, result : REAL;
   n : INTEGER;
   ok, again : BOOLEAN;
   proc : PROCEDURE(REAL,INTEGER,VAR BOOLEAN):REAL;
   smb : CHAR;

PROCEDURE stepen1(a: REAL; n: INTEGER; VAR ok: BOOLEAN): REAL;
CONST
     maxr = MAX(REAL);
VAR
   stepen : REAL;
BEGIN
    IF (a = 0.0) AND (n <= 0) THEN
      ok := FALSE;
      RETURN 0.0
    ELSE
        ok := TRUE;
        IF (a = 0.0) THEN
          RETURN 0.0
        ELSIF (a = 1.0) OR (n = 0) THEN
             RETURN 1.0
        ELSIF n = 1 THEN
             RETURN a
        ELSE
            IF n < 0 THEN
              a := 1.0 / a;
              n := ABS(n)
            END;
            stepen := 1.0;
            WHILE (n > 0) AND ok DO
                 IF ODD(n) THEN
                   stepen := stepen * a
                 END;
                 n := n DIV 2;
                 ok := (maxr / a) > a;
                 IF (n > 0) AND ok THEN
                   a := a * a
                 END
            END;
            RETURN stepen;
         END
     END;
END stepen1;


PROCEDURE stepen2(a: REAL; n: INTEGER; VAR ok: BOOLEAN): REAL;

        PROCEDURE Sqr(x: REAL): REAL;
        BEGIN
             RETURN x*x
        END Sqr;

        PROCEDURE RekStepen(a: REAL; n: INTEGER): REAL;
        BEGIN
             IF n = 1 THEN
               RETURN a
             ELSIF ODD(n) THEN
                  RETURN a * Sqr(RekStepen(a,n DIV 2))
             ELSE
                 RETURN Sqr(RekStepen(a,n DIV 2))
             END;
        END RekStepen;

BEGIN
     IF (a = 0.0) AND (n <= 0) THEN
       ok := FALSE;
       RETURN 0.0
     ELSE
         ok := TRUE;
         IF a = 0.0 THEN
           RETURN 0.0
         ELSIF (n = 0) OR (a = 1.0) THEN
              RETURN 1.0
         ELSIF n = 1 THEN
              RETURN a
         ELSE
             IF n < 0 THEN
               a := 1.0 / a;
               n := ABS(n)
             END;
             RETURN RekStepen(a, n)
         END
     END
END stepen2;

BEGIN
    WriteString('Enter a: ');
    ReadReal(a);
    WriteLn;
    WriteString('Enter n: ');
    ReadInt(n);
    WriteLn;
    WriteString('Chose your procedure');
    WriteLn;
    WriteString(' Enter i for iteratively solution;');
    WriteLn;
    WriteString(' or r for recursively solution (same result ofc)!'); WriteLn;
    again := FALSE;
    REPEAT
          IF again THEN
            WriteString('Enter again, input must be <i> OR <r> : ');
            WriteLn;
          END;
          Read(smb);
          WriteLn;
          again := TRUE;
     UNTIL (smb = 'i') OR (smb = 'r');
     CASE smb OF
         'i' : proc := stepen1|
         'r' : proc := stepen2
     END;
     result := proc(a,n,ok);
     IF ok THEN
        WriteString('Result for a^n is: ');
        WriteReal(result,10)
     ELSE
         WriteString('Invalid input')
     END
END z.