Getting the possible stack states at the call site.

CreateH.cs

ImmutableHashSet<EvalStack> GetPossibleStacksAt(Instruction call)
{
  var h = CreateH(call);
  return _I.Select(h).ToImmutableHashSet();
} 

Func<EvalStack, EvalStack> CreateH(Instruction call)
{
  return CreateComposite(_first, call.Previous);
}