In particular the zsh syntax
arc diff -F <(some command) creates a named pipe and passes it as a file reference:
[10021 eax@EagleMac ~ ]%file <(echo foo) /dev/fd/11: fifo (named pipe) [10023 eax@EagleMac ~ ]%cat <(echo foo) foo
but arc dies when reading this:
Requested path `/dev/fd/11' is not a file.
A named pipe should be allowed