Data::Hopen::Tool - Base class for packages that know how to process files
A tool knows how to generate a command or other text that will cause a build system to perform a particular action on a file belonging to a particular language.
A tool is an App::hopen::G::Cmd, so may interact with the current generator ("$Generator" in App::hopen::BuildSystemGlobals). Moreover, the generator will get a chance to visit the op after it is processed.
Maybe TODO: - Each Generator must specify a list of Content-Types (media types) it can consume. Each Tool must specify a specific content-type it produces. Mismatches are an error unless overriden on the hopen command line.