def coalesce( other )
      @sum += other.sum
      @sumsq += other.sumsq
      if other.num > 0
        @min = other.min if @min > other.min
        @max = other.max if @max < other.max
        @last = other.last
      end
      @num += other.num
    end
def coalesce( other )
      @sum += other.sum
      @sumsq += other.sumsq
      if other.num > 0
        @min = other.min if @min > other.min
        @max = other.max if @max < other.max
        @last = other.last
      end
      @num += other.num
    end
Coalesce the statistics from the _other_ sampler into this one. The
 _other_ sampler is not modified by this method.

 Coalescing the same two samplers multiple times should only be done if
 one of the samplers is reset between calls to this method. Otherwise
 statistics will be counted multiple times.