type letter = A | B | C | D | F

logic marks: int
logic grade: letter

goal start:
(marks=75  ->
     ((marks>30)  ->
          ((marks>50)  ->
               ((marks>70)  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         B=B)
)
 and 
               (not((marks>70))  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         C=B)
)
)
 and 
          (not((marks>50))  ->
               ((marks>70)  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         B=B)
)
 and 
               (not((marks>70))  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         D=B)
)
)
)
 and 
     (not((marks>30))  ->
          ((marks>50)  ->
               ((marks>70)  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         B=B)
)
 and 
               (not((marks>70))  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         C=B)
)
)
 and 
          (not((marks>50))  ->
               ((marks>70)  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         B=B)
)
 and 
               (not((marks>70))  ->
                    ((marks>90)  ->
                         A=B)
 and 
                    (not((marks>90))  ->
                         F=B)
)
)
)
)