The sqrt filter is a numeric filter whose value is the integer square root of its argument:
    sqrt 4  2
    sqrt 26  5
If the argument to the sqrt filter does not match the currentposition, neither does the sqrt filter. If the argument to sqrt is negative, the sqrt filter does not match the position.

The sqrt filter is mainly used in computing the Euclidean distance between two squares s and t, but the distance has to be scaled because CQL only has integers. See the cql file averagedistance.cql for an example of this usage.