The first line of a trajectory file defines the number of comment records. This is followed by ``header" records.
CORD 1000 1000 0 5932 0.204548E-01
^ type ^begin ^skip ^void ^number of atoms
^ timestep
10000.0 800.000 12Z6
^ scale ^ offset ^ format
174 1 21 23 27 28 39
^number of free atoms
^ ^ ^ ^ ^ ... Ids of free atoms
Header records are in turn followed by coordinate sets separated by the
string CORD.
The first coordinate set contains
all atoms. The subsequent coordinate sets
contain only the selected atoms (see, for example,
the CSELection statement in Section 10.1.6).