systemd redshift and xdm-archlinux

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https://bbs.archlinux.org – I can get redshift to start with modification to xdm-archlinux configuration scripts, but everytime I log out of the window manager a redshift stays in memory, and when it again runs xdm-archlinux, another redshift starts in tandem.  I wish there was a better way to control redshift so that after logging into the window manager/desktop environment, it will close the old redshift and start the new one.  I think I could do this with creating a redshift.service but could it also be done by just adding another ExecStart line added to xdm-archlinux.service?I want redshift to basically be (HowTos)