Error: Assertion failed: robotic arm is constrained to extend from right to left
at window.assertions.assertFunction (http://phettest.colorado.edu/assert/js/assert.js:23:13)
at http://phettest.colorado.edu/hookes-law/js/common/model/RoboticArm.js?bust=1475600956672:32:17
at Property.link (http://phettest.colorado.edu/axon/js/Property.js?bust=1475600956672:209:11)
at new RoboticArm (http://phettest.colorado.edu/hookes-law/js/common/model/RoboticArm.js?bust=1475600956672:31:23)
at new SingleSpringSystem (http://phettest.colorado.edu/hookes-law/js/common/model/SingleSpringSystem.js?bust=1475600956672:31:23)
at new IntroModel (http://phettest.colorado.edu/hookes-law/js/intro/model/IntroModel.js?bust=1475600956672:25:20)
at IntroScreen.createModel (http://phettest.colorado.edu/hookes-law/js/intro/IntroScreen.js?bust=1475600956672:34:27)
at IntroScreen.initializeModel (http://phettest.colorado.edu/joist/js/Screen.js?bust=1475600956672:182:26)
at IntroScreen.initializeModelAndView (http://phettest.colorado.edu/joist/js/Screen.js?bust=1475600956672:208:12)
at initializeScreen (http://phettest.colorado.edu/joist/js/Sim.js?bust=1475600956672:436:14)