From a42e99e28d22bd0a313d4bac23cd4278627be1a3 Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Tue, 12 Nov 2019 20:36:43 -0500
Subject: [PATCH] New \restrict macro to restrict a function to a subset of its
 domain.

---
 examples.tex   |  7 ++++---
 mjo-common.tex | 17 +++++++++++++++++
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/examples.tex b/examples.tex
index 3a1d6c4..c795973 100644
--- a/examples.tex
+++ b/examples.tex
@@ -72,9 +72,10 @@
   \end{section}
 
   \begin{section}{Common}
-    The function $f$ applied to $x$ is $f\of{x}$. We can group terms
-    like $a + \qty{b - c}$ or $a + \qty{b - \sqty{c - d}}$. The tuples
-    go up to seven, for now:
+    The function $f$ applied to $x$ is $f\of{x}$, and the restriction
+    of $f$ to a subset $X$ of its domain is $\restrict{f}{X}$. We can
+    group terms like $a + \qty{b - c}$ or $a + \qty{b - \sqty{c -
+        d}}$. The tuples go up to seven, for now:
     %
     \begin{itemize}
       \begin{item}
diff --git a/mjo-common.tex b/mjo-common.tex
index 38b262b..284d5a5 100644
--- a/mjo-common.tex
+++ b/mjo-common.tex
@@ -8,6 +8,10 @@
   \usepackage{amsfonts}
 \fi
 
+\ifx\restriction\undefined
+  \usepackage{amssymb}
+\fi
+
 % Place the argument in matching left/right parentheses.
 \newcommand*{\of}[1]{ \left({#1}\right) }
 
@@ -38,6 +42,19 @@
 % The factorial operator.
 \newcommand*{\factorial}[1]{ {#1}! }
 
+% Restrict the first argument (a function) to the second argument (a
+% subset of that functions domain). Abused for polynomials to specify
+% an associated function with a particular domain (also its codomain,
+% in the case of univariate polynomials).
+\newcommand*{\restrict}[2]{{#1}{\restriction}_{#2}}
+\ifdefined\newglossaryentry
+  \newglossaryentry{restriction}{
+    name={\ensuremath{\restrict{f}{X}}},
+    description={the restriction of $f$ to $X$},
+    sort=r
+  }
+\fi
+
 %
 % Product spaces
 %
-- 
2.43.2