This code is originally a fork from the Fast Downward planning system and is
augmented with the fd-hc-refinement algorithm and generation of a proof for it.

The fd-hc-refinement algorithm was implemented by Marcel Steinmetz.

Our contribution for the ICAPS 2018 paper "A Proof System for Unsolvable
Planning Tasks" is highlighted in the code with the comments "// begin
certifying-fd-hc-refinement" and "// end certifying-fd-hc-refinement".

