docs: function( v, k, m, s ){
  
              var x = v - m;
  
              // Mk = Mk-1+ (xk – Mk-1)/k
              // Sk = Sk-1 + (xk – Mk-1)*(xk – Mk).
              m += x / k;
              s += x * (v - m);
              return [m, s];
          }