From: Michael Orlitzky <michael@orlitzky.com>
Date: Fri, 2 May 2025 15:58:49 +0000 (-0400)
Subject: {examples,mjo-arrow}.tex: support blank \identity argument
X-Git-Url: http://gitweb.michael.orlitzky.com/?a=commitdiff_plain;h=8395ab5f5ad59b35292f22506ee67190004f102a;p=mjotex.git

{examples,mjo-arrow}.tex: support blank \identity argument
---

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}{