type expr = 
    Cst of int
  | Var of string
  | Sum of expr*expr
  | Diff of expr*expr 
  | Prod of expr*expr
  | Div of expr*expr
  | Letin of string*expr*expr

	  
type instr = 
    Set of string*expr 
  | Print of expr


val read_expr : string -> expr
val read_instr : string -> instr