Add XKCD 687 example

examples/xkcd_687.nbt

@@ -0,0 +1,12 @@
+# Dimensional analysis
+#
+# https://xkcd.com/687/
+#
+
+let earth_core_pressure = 3.5 million atmospheres
+let prius_milage = 50 miles per gallon
+let english_channel_min_width = 21 miles
+
+let c: Scalar = planck_energy / earth_core_pressure * prius_milage / english_channel_min_width -> 1  # TODO: get rid of the conversion here
+
+print(c)