Arithmetic expressions example from section

To build it:
  $ cp arithParser1.mly arithParser.mly
  $ make clean
  $ make

To test it:
  $ ./arith.top
  # open Arith;;
  
  # eval_string [] "2*3+5";;
  # eval_string [("x", 500)] "x / 10";;
  
  # let exp1 = parse_string "2*3+5";;
  # Arith.eval [] exp1;;
  
  # let exp2 = parse_string "x / 10";;
  # Arith.eval [("x", 500)] exp2;;