def __init__(self, f, sub=slice(None)):
SlicedReader.__init__(self, f, sub)<EOL>self.num_atoms = None<EOL>pos = self._read_frame()[<NUM_LIT:1>]<EOL>self.num_atoms = len(pos)<EOL>self._f.seek(<NUM_LIT:0>)<EOL>
Argument:
 | ``f``  --  a filename or a file-like object

Optional argument:
 | ``sub``  --  a slice object to indicate the frames to be read/skipped.