Update style guide on the use of \firstuse

styleguide.md

@@ -155,9 +155,12 @@ base-clock | `base-clock` | Similarly: sub-cock
 
 New terminology is either introduced with a `definition` environment, or as part of the running text.
 When part of the running text, the introduced terminology should be marked with `\firstuse` at the point of the definition.
-As a general rule, `\firstuse` should appear in combination with `\index` for adding the term to the document index.
-(The use of `\firstuse` instead of just `\emph` helps us both produce consistent formatting and makes it easier to spot cases where the additional use of `\index` has been forgotten.
-The reason that `\firstuse` doesn't also do the job calling `\index` is that the form of the term presented to `\firstuse` isn't always in the base form expected in the document index, that there can be a need for special styling tricks in the `\index` argument, etc.)
+As a general rule, terminology introduced with `\firstuse` should appear in the document index, and by default the mandatory argument to `\firstuse` is automatically passed to `\index`.
+To change the appearance of the index entry, the default may be overridden using an optional argument to `\firstuse`, for example, `\firstuse[array!variable]{array variable}`.
+This is also useful when capitalization or plural/singular differs; excapt for things like names, lower case should be used in the index, and terms should typically appear in the singular, for example, `\firstuse[vector]{Vectors}`.
+On rare occasions, one just wants the standardized typesetting of `\firstuse` but no entry in the index, which can be achieved by passing an em-dash for the optional argument, for example, \firstuse[---]{constant}.
+When suppressing the appearnce in the index, it is recommended to add a comment in the source explaining why.
+It is common that the use of `\firstuse` is directly followed by additional calls to `\index` for adding the terminology in more variants to the document index.
 
 If the new terminology is used before being introduced, it should be marked with `\willintroduce` (instead of `\firstuse`) to alert the reader that this is not a term that is expected to be known yet by a first-time reader.