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§27.8. Extensions can include other extensions
Extensions can themselves contain "Include..." sentences asking for other extensions to be included. An extension might, for example, start like this:
A project which asks to include "Basic Help Menu" will then also include "Menus", even though the author might never even realise that. Indeed, the author could also have asked to include "Menus", not realising that "Basic Help Menu" was going to ask for the same thing.
So the same extension is often requested multiple times. This is fine if the version numbers in the requests are compatible, but they might not be. For instance, suppose the main source text asks to include version 2 of extension X, and also to include extension Y. Suppose further that Y contains a request to include version 4 of X. We now have two different requests for X, and they contradict each other - the major version of X cannot be both 2 and 4 at the same time. So Inform will produce a problem message in this case.
But in cases where it is possible for everyone to be satisfied, Inform will try to find a solution. If one extension asks for version 2.3 of X, and another asks just for X, and a third asks for version 2.7.2 of X, then Inform will work out that any version number in the range 2.7.2 up to (but not including) 3 will be fine. If it can in fact find such an extension, it will then use it. So if the user has version 2.8.17 installed, everything is fine.
If an extension does include other extensions, it is good style to place the "Include..." sentence(s) as early as possible after the introductory sentence, just so that human readers looking at the text of the extension can see these dependencies easily.
