From 339be9b2d259eb7926d5d6d2065056c5f131a163 Mon Sep 17 00:00:00 2001
From: Domenic Denicola <d@domenic.me>
Date: Fri, 3 May 2019 16:58:30 -0400
Subject: [PATCH] Add "starts with" synonym for "code unit prefix"

---
 infra.bs | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/infra.bs b/infra.bs
index 99b8f3c..c4c3b47 100644
--- a/infra.bs
+++ b/infra.bs
@@ -543,37 +543,49 @@ actually ends up representing <a lt="JavaScript string">JavaScript</a> and
 <a>scalar value strings</a>. It is even fairly typical for implementations to have multiple
 implementations of just <a>JavaScript strings</a> for performance and memory reasons.
 
-<p>A <a>string</a> <var>a</var> is a <dfn export>code unit prefix</dfn> of a <a>string</a>
-<var>b</var> if the following steps return true:</p>
+<hr>
+
+<p>A <a>string</a> <var>a</var> is a
+<dfn export lt="code unit prefix|starts with">code unit prefix</dfn> of a <a>string</a> <var>b</var>
+if the following steps return true:
 
 <ol>
- <li><p>Let <var>i</var> be 0.</p></li>
+ <li><p>Let <var>i</var> be 0.
 
  <li>
-  <p><a>While</a> true:</p>
+  <p><a>While</a> true:
 
   <ol>
    <li><p>Let <var>aCodeUnit</var> be the <var>i</var>th <a>code unit</a> of <var>a</var>, or null
-   if <var>i</var> is greater than or equal to <var>a</var>'s <a for=string>length</a>.</p></li>
+   if <var>i</var> is greater than or equal to <var>a</var>'s <a for=string>length</a>.
 
    <li><p>Let <var>bCodeUnit</var> be the <var>i</var>th <a>code unit</a> of <var>b</var>, or null
-   if <var>i</var> is greater than or equal to <var>b</var>'s <a for=string>length</a>.</p></li>
+   if <var>i</var> is greater than or equal to <var>b</var>'s <a for=string>length</a>.
 
-   <li><p>If both <var>aCodeUnit</var> and <var>bCodeUnit</var> are null, then return true.</p></li>
+   <li><p>If both <var>aCodeUnit</var> and <var>bCodeUnit</var> are null, then return true.
 
    <li><p>If <var>aCodeUnit</var> is null and <var>bCodeUnit</var> is non-null, then return
-   true.</p></li>
+   true.
 
    <li><p>If <var>aCodeUnit</var> is non-null and <var>bCodeUnit</var> is null, then return
-   false.</p></li>
+   false.
 
-   <li><p>Return false if <var>aCodeUnit</var> is different from <var>bCodeUnit</var>.</p></li>
+   <li><p>Return false if <var>aCodeUnit</var> is different from <var>bCodeUnit</var>.
 
-   <li><p>Set <var>i</var> to <var>i</var> + 1.</p></li>
+   <li><p>Set <var>i</var> to <var>i</var> + 1.
   </ol>
  </li>
 </ol>
 
+<p>When it is clear from context that <a>code units</a> are in play, e.g., because one of the
+strings is a literal containing only characters that are in the range U+0020 SPACE to U+007E (~),
+"<var>a</var> starts with <var>b</var>" can be used as a synonym for "<var>b</var> is a
+<a>code unit prefix</a> of <var>a</var>".
+
+<p class=example id=code-unit-prefix-example>With unknown values, it is good to be explicit:
+<var>targetString</var> is a <a>code unit prefix</a> of <var>userInput</var>. But with a literal,
+we can use plainer language: <var>userInput</var> starts with "<code>!</code>".
+
 <p>A <a>string</a> <var>a</var> is <dfn export>code unit less than</dfn> a <a>string</a>
 <var>b</var> if the following steps return true: