some (very little) work on commenting

src/rustc/middle/borrowck/categorization.rs

@@ -7,7 +7,15 @@ determine what kind of memory is used in evaluating it (for example,
 where dereferences occur and what kind of pointer is dereferenced;
 whether the memory is mutable; etc)
 
+Categorization effectively transforms all of our expressions into
+expressions of the following forms (the actual enum has many more
+possibilities, naturally, but they are all variants of these base
+forms):
 
+    E = rvalue    // some computed rvalue
+      | x         // address of a local variable, arg, or upvar
+      | *E        // deref of a ptr
+      | E.comp    // access to an interior component
 
 Imagine a routine ToAddr(Expr) that evaluates an expression and returns an
 address where the result is to be found.  If Expr is an lvalue, then this