The file filename must be an ascii file.
Marked particles appear green in plots.
Particles in a particular box can be unmarked using the unmarkbox command and all the marked particles can be unmarked using the unmarkall command. Particles remain marked even when a new time frame is loaded. Particles can also be marked using the markarray, markbox, and markgal commands. Statistics about marked particles can be determined using the markstat command. Marked particles can be written to a file using the writemark command.