Returns a map of the distinct values of ks in the xrel mapped to a
set of the maps in xrel with the corresponding values of ks.
Source
(defnindex"Returns a map of the distinct values of ks in the xrel mapped to a\n set of the maps in xrel with the corresponding values of ks."[xrelks](reduce(fn[mx](let[ik(select-keysxks)](assocmik(conj(getmik#{})x)))){}xrel))