Tmux: map CAPS to ctrl-b prefix with Seil/Karabiner

private.xml

<?xml version="1.0"?>
<root>
  <!-- Thanks Brett Terpstra for the tip -->
  <!-- http://brettterpstra.com/2012/12/08/a-useful-caps-lock-key/ -->
	<item>
		<name>Tmux Prefix</name>
		<appendix>CAPS to control-b</appendix>
		<identifier>private.tmux_prefix</identifier>
		<autogen>
			--KeyOverlaidModifier--
			KeyCode::F19,
			KeyCode::B,
			ModifierFlag::CONTROL_L
		</autogen>
	</item>
</root>