I handled your comments, merged the code and pushed to the master repo.

LGTM. I added a few more comments on bitbucket.

Thanks for your comments, Malte! I agree with all of them and handled them in a 
single commit.

Okay, I'll wait anyways until you have updated the pull request. I just wanted
to look at the code out of curiosity, since I also implemented this for the IPC.

Silvan, I don't think a second review is necessary now that Malte had a look at 
the code, but of course you're more than welcome to make comments :-)

I'll also have a look once I find some time to do so.

I left some code review comments.

Here are the results for an experiment evaluating the current patch.

http://ai.cs.unibas.ch/_tmp_files/seipp/issue773-v1-opt-issue773-base-issue773-v1-compare.html

The left comparison compares plain simple stubborn sets and simple stubborn sets with 
min_pruning_ratio=0.2, i.e., pruning is diabled after 1000 expansions if not at least 0.2 of 
the successors have been pruned (for simplicity the two configs use the same config and the 
parser of the base version just ignores the unknown min_pruning_ratio argument). The right 
comparison checks that the plain simple stubborn sets (without min_pruning_ratio) are not 
affected by the patch.

Using min_pruning_ratio=0.2 raises the coverage of LM-cut by 11 tasks. 8 domains are affected 
positively by the change while coverage decreases by 1 in Rovers. The patch doesn't seem to 
affect the code path taken without min_pruning_ratio.

Silvan, could you please review the code?

I made a pull request at
https://bitbucket.org/jendrikseipp/downward/pull-requests/81 .