   stroke: (strokableThings) =>
     @context.beginPath()
     for thing in strokableThings
+      continue unless thing
       for name, args of thing
         if name is "rectangle"
           @rectanglePath.apply @, args


  polyline: (vectors) =>
    for vector, i in vectors
      if i == 0
        @context.moveTo.apply @context, vector
      else

  parse: (commands) =>
    return unless @context
    for command in commands
      continue unless command
      for instr, args of command
        if @[instr]?
          @[instr].apply @, [args]
