Note, in the explict form:
PythonInstalled.new('2.7') => :recommended
the tag :recommended is ignored (not a limitation
of PythonInstalled itself). One solution was to write
PythonInstalled.new('2.7', [:recommended])
but that is not as beautiful as we like it.
Therefore, now it is possible to:
depends_on :python => ['2.7', :recommended]
Only the first tag is attempted to be parsed as
a version specifyer "x" or "x.y" or "x.y.z"...
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