note a few suspect areas of code

crates/coremem/src/sim/mod.rs

@@ -143,7 +143,13 @@ impl<'a, R: Real, M: Material<R>> Sample<'a, R, M> {
     }
 
     pub fn current_density(&self) -> Vec3<R> {
-        // TODO: does this make sense for Pml?
+        // TODO: justify/derive this.
+        // i guess the actual current density is the gradient of E multiplied by conductivity,
+        // and that when summed over a loop the negative terms from the neighbors are canceled
+        // when we grab their current density?
+        //
+        // or maybe it's $V = \int{E . dl} = IR$, so $I = \int{\sigma E . dl}$
+        // in which case this might not be "current density", but something related.
         let conductivity = self.conductivity();
         self.e().elem_mul(conductivity)
     }