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Parsing expressions

This preliminary example can now be improved to other grammars.

EXP = NUM ('+' NUM | epsilon)

let exp = 
  num'       && fun n1 -> 
  ((sym '+') && fun _  -> 
   num'      && fun n2 -> 
   ret (n1+n2)) 
  || (ret n1);;

exp "12";;
exp "12+34";;

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