Maybe italicize, boldface, or System style the filename to make ti stand out. I explain it to my students as a dump of memory of the Squeak virtual machine. In Squeak, the left side is not actually evaluated. Squeak has no explicit destructor methods, true, but there are ways of getting that with Weak Arrays. Object doesNotUnderstand occurs at the end of the process.
Length is immutable.
Asking a bag if it holds an object takes almost no time at all. I'm thinking of a tiny tutorial on the debugger, or a picture and description of the MethodFinder.