Composing g and h functions from f-functions.

CreateG.cs

private Func<EvalStack, EvalStack> CreateG()
{
  return CreateComposite(_first, _last);
}

private Func<EvalStack, EvalStack> CreateComposite(
  Instruction first, 
  Instruction last)
{
  Instruction insn = first;
  Func<EvalStack, EvalStack> fun = stack => stack;
  while (insn != null)
  {
    var f = CreateF(insn);
    var fresh = fun;
    fun = stack => f(fresh(stack));
    insn = (insn == last) ? null : insn.Next;
  }

  return fun;
}