CR_INVERT flag?

I'm pulling myself up the learning curve for Comedi reading the
documentation and the demo programs.  I'm stuck finding the meaning of
CR_INVERT flag.  It is used in one of the demo programs, receiver.c: (the
comments are mine as I try to grasp the program.

cmd->scan_begin_src =    TRIG_EXT;   //Start a scan from an external trigger
cmd->scan_begin_arg =    CR_EDGE | CR_INVERT | pin_clk;  //

pin_clk is the pin for the external trigger, CR_EDGE is self explanatory (I
think).  Does CR_INVERT mean it triggers only on falling edge?

Sorry for a basic question but I have searched the documentation to no
avail.
David