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)
     }

crates/coremem/src/stim.rs

@@ -150,6 +150,7 @@ impl<R: Region + Sync, T: TimeVarying1 + Sync> AbstractStimulus for CurlStimulus
         if self.region.contains(pos) {
             let (amt_e, amt_h) = self.stim.at(t_sec);
             let from_center_to_point = *pos - *self.center;
+            // TODO: is this inverted?
             let rotational = from_center_to_point.cross(*self.axis);
             let impulse_e = rotational.with_mag(amt_e.cast()).unwrap_or_default();
             let impulse_h = rotational.with_mag(amt_h.cast()).unwrap_or_default();