Services
A service is a feature provided by a plugin. A typical service would be a function to build terms, or to traverse all sub-terms, or to perform E-matching, etc.
A registry is used to list all services provided by plugins and make them available to users of the library, or to other plugins.
The way to access a service registered by some plugin
Sourcetype any = | Any : 'a Key.t * 'a -> anyExistential wrapper around a service