Define string concatenation

Fixes #185.

infra.bs

@@ -486,6 +486,24 @@ implementations of just <a>JavaScript strings</a> for performance and memory rea
 
 <hr>
 
+<p>To <dfn export for=string lt=concatenate|concatenation>concatenate</dfn> a <a for=/>list</a> of
+<a for=/>strings</a> <var>list</var>, with an optional separator string <var>separator</var>, run
+these steps:
+
+<ol>
+ <li><p>If <var>list</var> <a for=list>is empty</a>, then return the empty string.
+
+ <li><p>If <var>separator</var> is not given, then set <var>separator</var> to the empty string.
+
+ <li><p>Return a <a for=/>string</a> whose contents are <var>list</var>'s <a for=list>items</a>, in
+ order, separated from each other by <var>separator</var>.
+</ol>
+
+<p class=example id=example-string-concatenate>To serialize a set <var>set</var>, return the
+<a for=string>concatenation</a> of <var>set</var> with U+0020 SPACE.
+
+<hr>
+
 <p>To <dfn export>isomorphic encode</dfn> a <a>string</a> <var>input</var>, run these steps:</p>
 
 <ol>
@@ -1076,6 +1094,7 @@ Leonid Vasilyev,
 Malika Aubakirova,
 Michael™ Smith,
 Mike West,
+Ms2ger,
 Philip Jägenstedt,
 Rashaun "Snuggs" Stovall,
 Sergey Shekyan,