25.8. Extensions can interact with other extensions

| Chapter 25: Extensions |

| 25.8. Extensions can interact with other extensions

When one extension is being used, it's probably only one among several. A really general-purpose extension might want to behave differently depending on which other extensions are also present. This can be achieved using headings which are "for use with" (or "without") other extensions. For instance:

> Chapter 2a (for use with Locksmith by Emily Short)

specifies that everything under this heading (and its subheadings, if any) will be ignored unless the extension Locksmith by Emily Short is included. Conversely,

> Chapter 2b (for use without Locksmith by Emily Short)

will be ignored unless it isn't included. This allows an extension to give two variations on the same material - one if Locksmith is present, the other if not.

Headings can also replace portions of extensions which have been included. For instance:

> Section 6 - Hacked locking (in place of Section 1 - Regular locking in Locksmith by Emily Short)

places the source text under the new heading in the place of the old (which is thrown away). If there should be two or more headings of the same name in the given extension, the first is the one replaced; if two or more headings attempt to replace the same heading in the given extension, the final attempt in source text order is the one which succeeds; and finally, heading dependencies like the above are scanned in a top-down way. Thus, if we have:

> Chapter 2a (for use with Locksmith by Emily Short)
> ...
> Section 1 - Hacked marbles (in place of Section 4 in Marbles by Peter Wong)
> ...

and we don't include Locksmith, then the replacement of Section 4 of Marbles is not made, because Section 1 - Hacked marbles is subordinate to the Chapter 2a heading which we've told Inform to ignore.

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