cfunction
(file/seek f &opt whence n)
Jump to a relative location in the file. 'whence' must be one of
:cur - jump relative to the current file location
:set - jump relative to the beginning of the file
:end - jump relative to the end of the file
By default, 'whence' is :cur. Optionally a value n may be passed for
the relative number of bytes to seek in the file. n may be a real
number to handle large files of more the 4GB. Returns the file handle.