Reworked example

examples/earth_mass.ins

@@ -1,6 +1,6 @@
 # Compute the mass of planet earth
 
-let radius = 6378 * 1000 * meter
-let volume = 4/3 * π * radius^3
-let density = 5514 * kilogram / meter^3
+let radius: Length = 6378 * 1000 * meter
+let volume: Volume = 4/3 * π * radius^3
+let density: MassDensity = 5514 * kilogram / meter^3
 volume * density -> kilogram