From 8395ab5f5ad59b35292f22506ee67190004f102a Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Fri, 2 May 2025 11:58:49 -0400
Subject: [PATCH] {examples,mjo-arrow}.tex: support blank \identity argument
---
examples.tex | 7 ++++---
mjo-arrow.tex | 10 ++++++++--
2 files changed, 12 insertions(+), 5 deletions(-)
diff --git a/examples.tex b/examples.tex
index 31acc21..e9b409d 100644
--- a/examples.tex
+++ b/examples.tex
@@ -68,9 +68,10 @@
\begin{section}{Arrow}
The constant function that always returns $a$ is $\const{a}$. The
- identity operator on $V$ is $\identity{V}$. The composition of $f$
- and $g$ is $\compose{f}{g}$. The inverse of $f$ is
- $\inverse{f}$. If $f$ is a function and $A$ is a subset of its
+ identity operator on $V$ is $\identity{V}$ but the argument can be
+ left blank to get a ``generic'' identity, $\identity{}$. The
+ composition of $f$ and $g$ is $\compose{f}{g}$. The inverse of $f$
+ is $\inverse{f}$. If $f$ is a function and $A$ is a subset of its
domain, then the preimage under $f$ of $A$ is $\preimage{f}{A}$.
\end{section}
diff --git a/mjo-arrow.tex b/mjo-arrow.tex
index 2d5095e..634f436 100644
--- a/mjo-arrow.tex
+++ b/mjo-arrow.tex
@@ -23,8 +23,14 @@
}
\fi
-% The identity function/arrow on its argument.
-\newcommand*{\identity}[1]{ \operatorname{id}_{{#1}} }
+% The identity function/arrow on its argument. Leave the argument
+% blank to get the plain "id"
+\newcommand*{\identity}[1]{%
+ \operatorname{id}%
+ \if\relax\detokenize{#1}\relax\else%
+ _{#1}%
+ \fi%
+}
\ifdefined\newglossaryentry
\newglossaryentry{identity}{
--
2.49.0