MNT: Comment

splinterpolator.h

@@ -1684,6 +1684,9 @@ double Splinterpolator<T>::SplineColumn::init_fwd_sweep(double z, ExtrapolationT
     double z2i=z;
     for (unsigned int i=1; i<n; i++, ptr--, z2i*=z) iv += z2i * *ptr;
   }
+  // This approach is not ideal for Constant/Zeros,
+  // but has been in place since 2008, so preserved
+  // to keep outputs consistent.
   else if (et == Mirror || et == Constant || et == Zeros) {
     double *ptr=&_col[1];
     double z2i=z;
@@ -1721,6 +1724,7 @@ double Splinterpolator<T>::SplineColumn::init_bwd_sweep(double z, double lv, Ext
     }
     iv /= (z2i-1.0);
   }
+  // See comment in init_fwd_sweep
   else if (et == Mirror || et == Constant || et == Zeros) {
     iv = -z/(1.0-z*z) * (2.0*_col[_sz-1] - lv);
   }