Add testing is never complete slide GH-47

Author: pintergreg

lectures/15_testing.md

@@ -1431,6 +1431,17 @@ def test_withdraw():
 
 <!-- # fuzz testing (fuzzing) -->
 
+## testing is never complete
+
+::: {.wide-quote}
+> By its nature, testing is never complete.
+As the influential computer scientist Edsger Dijkstra put it, “**Testing shows the presence, not the absence of bugs.**”
+No quantity of tests can ever prove a package free of bugs.
+At best, they increase our confidence that the package works well in a wide range of important scenarios.
+>
+> -- Alan Donovan & Brian Kernighan, “The Go Programming Language” [@donovan2015go]
+
+:::
 
 # legacy code
 

lectures/references.bib

@@ -1124,3 +1124,9 @@ @book{fowler2018refactoring
   year={2018},
   publisher={Addison-Wesley Professional}
 }
+@book{donovan2015go,
+  title={The Go programming language},
+  author={Donovan, Alan AA and Kernighan, Brian W},
+  year={2015},
+  publisher={Addison-Wesley Professional}
+}