rankfilter is a numeric filter that takes a single set filter as argument:
rank a1 == 1 rank c3 == 3
If the set of squares represented by its argument contains a single square s, then the value of the
rank filter is the numeric value of the rank of s.
If the set of squares represented by its argument does not contain exactly one single square, then the
rank filter does not match the current position.
rankfilter is used in averagedistance.cql, chameleon.cql, flipverticalecho.cql, horizontallysymmetric.cql, platzwechselecho.cql, platzwechseln2.cql .