code: function (body1, body2, springLength, springWeight, springCoefficient) {
        if (!body1 || !body2) {
          throw new Error('Cannot add null spring to force simulator');
        }
  
        if (typeof springLength !== 'number') {
          springLength = -1; // assume global configuration
        }
  
        var spring = new Spring(body1, body2, springLength, springCoefficient >= 0 ? springCoefficient : -1, springWeight);
        springs.push(spring);
  
        // TODO: could mark simulator as dirty.
        return spring;
      }
docs: Adds a spring to this simulation.
  
  @returns {Object} - a handle for a spring. If you want to later remove
  spring pass it to removeSpring() method.