I am trying to make a GUI application in common lisp with `ltk`, and there is one thing I just cannot figure out. I know I can set options of `ltk` widgets with `configure`, but I cannot figure out a way to read the values. 

For example, I create an instance of a canvas with 

```
(make-instance 'canvas :width 400 :height 400)

```

Then I want to write a method that will use the width and height in some calculations. How do I access these?