message: Use const refs
diff: @@ -40,8 +40,8 @@ inline spectrum<T> fresnel_reflectance(
  dielectric_tag /* */,
  spectrum<T> const& etai,
  spectrum<T> const& etat,
  - T cosi,
  - T cost
  + T const& cosi,
  + T const& cost
  )
  {
  // approximation for s-polarized light (perpendicular)
  