The abs filter is a numeric filter whose value is the absolute value of its single numeric argument:
    abs -4 == 4
    abs power a - power A
If its argument does not match the current position, neither does the abs filter.


The abs filter is used in platzwechselecho.cql and platzwechseln2.cql to check that the knights which have interchanged positions have ranks and files that are at least 3 apart:
    abs (rank N1 - rank N2)>= 3
    abs (file N1 - file N2)>= 3