This is the libraries-only edition. It still features liberal use of Beats::from_double() but this is now
explicit and will be easier to locate the calls and remove them. Several classes that were using
Beats::to_double() have been (temporarily) made friends of Beats to allow them to keep using it,
pending the much more widespread redesigns of several structures. Once this is done, the friend
relationships can (mostly) be removed. It is expected the ARDOUR::Variant will need to continue
as a friend because it is used to pass beat counts to LV2 as doubles
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