%{ 
  open Expr
%}
%token <string> Id_token
%token Left_parenthesis Right_parenthesis
%token Times_token Divide_token
%token Plus_token Minus_token
%token EOL
%start main
%type <Expr.expr> main
%%
expr:
   term                       { Term_as_Expr $1 }
 | term Plus_token expr	      { Plus_Expr ($1, $3) }
 | term Minus_token expr      { Minus_Expr ($1, $3) }
term:
   factor                     { Factor_as_Term $1 }
 | factor Times_token term    { Mult_Term ($1, $3) }
 | factor Divide_token term   { Div_Term ($1, $3) }
factor:
   Id_token                   { Id_as_Factor $1 }
 | Left_parenthesis expr Right_parenthesis
                              {Parenthesized_Expr_as_Factor $2 }
main:
 | expr EOL                   { $1 }