public interface IRangeSetUser level interface to RangeSet. Proposed rules for ranges: - By default (no arguments) RangeSet object is created with one valid range: (-infinity, +infinity). - size=0 means no valid ranges (isInRange(double point) will return false for any point). It also can be used to check if any range is set - RangeSet include valid interval only if size!=0. - RangeSet can be smart, e.g merge overlapping valid ranges. So size() does not always equal to the number of times user call include method.