Symbols
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- Must have LHS = # RHS is optional
- Leading/trailing space around LHS/RHS ignore, within strings, preserved
- Strings interpolated based on symbols seen *so far*, line by line
- Can be used on both hostlistfiles and command files

Test Mode
---------

- Default to test mode.  Use -t to turn off and actually execute.

- Shows what the program WOULD do after all includes and 
  variable substitutions are resolved.  Ditto the list of
  hosts involved.