Meaning/usage of $_ in bash

[Michael ODonnell]

OK - I now believe all observed behaviors are explainable
if that passage from TFM:

  "When bash invokes an external command, the variable
   $_ is set to the full file name of the command
   and passed to that command in its environment."

...is augmented thus:

  "Internally, bash uses $_ as essentially an
   alternate form of !$, which expands to the last
   token of the last command line."

Thanks to all.  As I said, it was just a nit...