Indexing commands in Eplain come in pairs: one command that only writes the index entry to the ‘.idx’ file (see above section), and one that also typesets the term being indexed. The former always starts with ‘s’ (for “silent”). In either case, the name always includes ‘Idx’, where I is the index letter, also described above. Eplain defines the index ‘i’ itself, so that's what we'll use in the names below.
The silent form of the commands take a subterm as a trailing optional
argument. For example, \sidx{truth}[definition of] on page 75
makes an index entry that will eventually be typeset (by default) as
truth
definition of, 75
Also, the silent commands ignore trailing spaces. The non-silent ones do not.