TapeContext::~TapeContext() {
  // TODO(srbs): Make AbstractContext ref counted.
  // parent_ctx_->Unref();
}
TapeOperation* TapeContext::CreateOperation() {
  return new TapeOperation(parent_ctx_->CreateOperation(), tape_, registry_);
}
Status TapeContext::RegisterFunction(AbstractFunction* f) {
  return parent_ctx_->RegisterFunction(f);
}
Status TapeContext::RemoveFunction(const string& func) {
  return parent_ctx_->RemoveFunction(func);
}