  The field of Grammatical Error Correction (GEC) has produced various systems
to deal with focused phenomena or general text editing. We propose an automatic
way to combine black-box systems. Our method automatically detects the strength
of a system or the combination of several systems per error type, improving
precision and recall while optimizing $F$ score directly. We show consistent
improvement over the best standalone system in all the configurations tested.
This approach also outperforms average ensembling of different RNN models with
random initializations.
  In addition, we analyze the use of BERT for GEC - reporting promising results
on this end. We also present a spellchecker created for this task which
outperforms standard spellcheckers tested on the task of spellchecking.
  This paper describes a system submission to Building Educational Applications
2019 Shared Task: Grammatical Error Correction.
  Combining the output of top BEA 2019 shared task systems using our approach,
currently holds the highest reported score in the open phase of the BEA 2019
shared task, improving F0.5 by 3.7 points over the best result reported.
