LetExp([
 TypeDec[
  (tree,
   RecordTy[
    (key,true,int),
    (children,true,treelist)])],
 VarDec(d,true,SOME(int),
  IntExp(0)),
 TypeDec[
  (treelist,
   RecordTy[
    (hd,true,tree),
    (tl,true,treelist)])]],
 VarExp(
  SimpleVar(d)))