type base_type = UNIT | BOOL | INT | FLOAT;;

    type simple_type = 
       BASE of base_type 
     | ARROW of simple_type * simple_type 
     | PRODUCT of simple_type list;;
    
    type identifier = string;;

    type operator = 
       Not | IAdd | ISub | IMul | IDiv 
           | FAdd | FSub | FMul | FDiv;;

    type constant = 
       Unit 
     | Bool of bool
     | Int of int 
     | Float of float 
     | Op of operator;;

    type expression = 
       Const of constant 
     | Id of identifier 
     | App of expression * expression
     | If of expression * expression * expression
     | Tuple of expression list
     | Lambda of identifier * simple_type * expression
     | Let of identifier * expression * expression
     | Rec of identifier * simple_type * expression;;