adds a new sorting rule to collection
$coll->add("name","type");
Parameters:
clears collection (deletes all specified sorting rules).
$coll->clear();
Parameters:
check if there are some rules in collection for the specified field and returns related hash.
$index = $coll->index("name");
Parameters:
Returns:
array of all specified sorting rules.
$name = $coll->rules[0]["name"]; $coll->rules[0]["direction"]="DESC";
Properties: