only perfom when output is missing

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