LetExp([
 FunctionDec[
  (do_nothing1,[   
   (a,true,int),
   (b,true,string)],
NONE,
   CallExp(do_nothing2,[
    OpExp(PlusOp,
     VarExp(
      SimpleVar(a)),
     IntExp(1))])),
  (do_nothing2,[   
   (d,true,int)],
NONE,
   CallExp(do_nothing1,[
    VarExp(
     SimpleVar(d)),
    StringExp("str")]))]],
 CallExp(do_nothing1,[
  IntExp(0),
  StringExp("str2")]))