An instance of t represents the identity of the contents of a file path. Use this to quickly detect if a file has changed. (Detection is done by checking some fields from stat syscall, it can be tricked but should behave well in regular cases). FIXME: precision of mtime is still the second?!

Returns true iff the heuristic determines that the file contents has not changed.

file_id filename computes an id for the current contents of filename. Returns a generic id, if the id can't be computed.

Same as get, but returns an error, if the id can't be computed.