~/X11/mptk-code14/ch01$ perl hello-world
X Error of failed request:  BadAtom (invalid Atom parameter)
  Major opcode of failed request:  18 (X_ChangeProperty)
  Atom id in failed request:  0xda
  Serial number of failed request:  12
  Current serial number in output stream:  15
~/X11/mptk-code14/ch01$ 

##</code><code>##

Atoms...map strings to an integer identifier...