only perfom when the output is missing

[Attila Lendvai]

dear hackers,

i cannot convince ASDF to only perform my custom operation if the output
file doesn't exist; i.e. regardless of the modification times.

it's in cffi/c2ffi: foo.h -> foo.spec -> foo.lisp

when the spec is there, regardless of the mod time, i'd like to skip the
spec generation task.

neither reading the manual, nor grepping around helped me to come up with
the required magic.

my custom operation-done-p isn't even called if the file's mod time
instructs ASDF to run the operation.

any hints are appreciated, including pointers to examples or tests.

- attila

PS: i sent the same mail from the wrong account, don't allow that through
moderation please.
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