Oh, wait: I'm confused.
What really happened for me: I saw this error and added what seemed to
be the obvious default argument (#f) to hash-ref on line 273 to make
this line:
(not (hash-ref simultaneous-installs (path->pkg f
#:cache path-pkg-cache) #f))))
in commit 48992001. Do you have that commit?
(After I added that commit, I got an error about a pkg conflict which
was the one avoided by deleting compiled/ directories).
Robby