xmodmap service documentation excerpt

     The ‘key-map’ field takes a list of objects, each of which is
     either a “statement” (a string) or an “assignment” (a pair of
     strings).  As an example, the snippet below swaps around the
     ‘Caps_Lock’ and the ‘Control_L’ keys, by first removing the keysyms
     (on the right-hand side) from the corresponding modifier maps (on
     the left-hand side), re-assigning them by swapping each other out,
     and finally adding back the keysyms to the modifier maps.

          (service home-xmodmap-service-type
                   (home-xmodmap-configuration
                    (key-map '(("remove Lock" . "Caps_Lock")
                               ("remove Control" . "Control_L")
                               ("keysym Control_L" . "Caps_Lock")
                               ("keysym Caps_Lock" . "Control_L")
                               ("add Lock" . "Caps_Lock")
                               ("add Control" . "Control_L")))))