thread-related patterns silently fail in fcc-hook
this works:
fcc-hook ~x. +foobar
this doesn't:
fcc-hook ~<(~A) +foobar
this cannot possibly work due to circularity, as the composed message cannot be threaded until it actually ends up in a particular fcc. the pattern matcher should verbosely fail (attempts to use ~h in that context yields "h: not supported in this mode", which seems adequate, though i don't see why that one in particular doesn't work). also, such limitations should be documented.