Improved usability for locating identifiers
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Perhaps something like this:

    \source context.slul has Ctx ctx_*
    \source tree.slul has Tr tr_*

Only the given identifiers will be available.
Attempting to make a non-local identifier with another suffix
should give an error.

A similar technique could be used for dependencies:

    \depends png 1.0 use Png png_*

This could be extended for renames

    \depends png 1.0 use Png=PngImage png_*=pngimage_*