code: function(derives, xStart, yStart, h) {
      this.derives    = derives
      this.x          = xStart
      this.y          = yStart || []
      this.dimension  = this.y.length
      this.h          = h || 0.01
  
      // cache the k1, k2, k3, k4 of each step
      this._k1
      this._k2
      this._k3
      this._k4
  }
docs: The fourth order Runge-Kutta integration method
  
  @class RungeKutta4
  @param {Function} derives Differential equations which needed to integrate
  @param {Number}   xStart  Initial value problem: x0
  @param {Array}    yStart  Initial value problem: y0
  @param {Number}   h       Each step-size value