 values 
have been assigned to the elements of array <CODE>F</CODE>. We first find the 
maximum and minimum values of <CODE>F</CODE>, and choose 10 contour levels evenly
spaced between the maximum and minimum:
<PRE>
      REAL F(50,50), ALEV(10), TR(6)
      ...
      FMIN = F(1,1)
      FMAX = F(1,1)
      DO 300 I=1,50
        DO 200 J=1,50
          FMIN = MIN(F(I,J),FMIN)
          FMAX = MAX(F(I,J),FMAX)
  200   CONTINUE
  300 CONTINUE
      DO 400 I=1,10
        ALEV(I) = FMIN + (I-1)*(FMAX-FMIN)/9.0
