code: function(s1,s2){
          var b1=s1.parent;
          var b2=s2.parent;
          if( b1==b2 || // same parents
              (!b1.isDynamic&&!b2.isDynamic) || // static or kinematic object
              (s1.belongsTo&s2.collidesWith)==0 ||
               (s2.belongsTo&s1.collidesWith)==0 // collision filtering
          ){ return false; }
          var js;
          if(b1.numJoints<b2.numJoints) js=b1.jointLink;
          else js=b2.jointLink;
          while(js!=null){
             var joint=js.joint;
             if( !joint.allowCollision && (joint.body1==b1&&joint.body2==b2 || joint.body1==b2&&joint.body2==b1) ){ return false; }
             js=js.next;
          }
          return true;
      }
docs: Returns whether the pair is available or not.
  @param   s1
  @param   s2
  @return