Plain TeX defines \active (as the number 13) for use in
changing category codes. Although the author of The TeXbook
has "intentionally kept the category codes numeric", two other
categories are commonly used: letters (category code 11) and others
(12). Therefore, Eplain defines \letter and
\other.
Sometimes it is cleaner to make a character active without actually
writing a \catcode command. The \makeactive command takes
a character as an argument to make active (and ignores following
spaces). For example, here are two commands which both make \
active:
\makeactive`\\ \makeactive92
Usually, when you give a definition to an active character, you have
to do so inside a group where you temporarily make the character active,
and then give it a global definition (cf. the definition of
\obeyspaces in The TeXbook).
This is inconvenient if you are writing a long macro, or if the
character already has a global definition you do not wish to transcend.
Eplain provides \letreturn, which defines the usual end-of-line
character to be the argument. For example:
\def\mymacro{... \letreturn\myreturn ... }
\mymacro hello
there
The end-of-line between `hello' and `there' causes
\myreturn to be expanded.
The TeXbook describes \uncatcodespecials,
which makes all characters which are normally "special" into "other"
characters, but the definition never made it into plain TeX.
Eplain therefore defines it.
Finally, \percentchar
expands into a literal `%' character. This is useful when you
\write TeX output to a file, and want to avoid spurious
spaces. For example, Eplain writes a \percentchar after the
definition of cross-references. The macros \lbracechar
and \rbracechar
expand similarly.