LetExp([
 VarDec(a,true,NONE,
  IntExp(0))],
 ForExp(
  i,true,
  IntExp(0),
  IntExp(100),
  SeqExp[
   AssignExp(
    SimpleVar(a),
    OpExp(PlusOp,
     VarExp(
      SimpleVar(a)),
     IntExp(1))),
   SeqExp[]]))