> As far as I can tell from Kernighan and Cherry[1], eqn's "define"s
> aren't parameterized at all. The groff eqn(1) man page claims to
> document (only) extensions to classical eqn, but I don't see this
> extension described there.
It actually does: in subsection "Macros":
Macros
Macros can take arguments. In a macro body, $n where
n is between 1 and 9, is replaced by the nth argument
if the macro is called with arguments; if there are
fewer than n arguments, it is replaced by nothing.
A word containing a left parenthesis where the part
of the word before the left parenthesis has been
defined using the define command is recognized as a
macro call with arguments; characters following the
left parenthesis up to a matching right parenthesis
are treated as comma-separated arguments; commas inside
nested parentheses do not terminate an argument.
So it does mention grouping with parentheses being understood,
but it says nothing about understanding "strings" delimited by
quotes.
