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This will eventually replace the Environment#vms method. Because of the introduction of providers, the environment doesn't know what the backing of the machines will be (and they're _machines_ now, not _vms_). Instead, users of Environment will now call `#machine` on the environment to retrieve a machine with the given backing provider as it needs it.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.