fixup! Lift restrictions for matching of binaries and maps

system/doc/reference_manual/expressions.xml

@@ -283,7 +283,9 @@ value
 3> <input>f(Key), #{Key := Value} = (#{key := Key} = M), Value.</input>
 value
 4> <input>f(Key), (#{Key := Value} = #{key := Key}) = M, Value.</input>
-* 1:12: variable 'Key' is unbound</pre>
+* 1:12: variable 'Key' is unbound
+5> <input>&lt;&lt;X:Y&gt;&gt; = begin Y = 8, &lt;&lt;42:8&gt;&gt; end, X.</input>
+42</pre>
 
     <p>The expression at prompt <em>2&gt;</em> first matches the value of
     variable <c>M</c> against pattern <c>#{key := Key}</c>, binding
@@ -307,6 +309,13 @@ value
     sub patterns are matched at the same time, and the variable
     <c>Key</c> is therefore not bound when matching against pattern
     <c>#{Key := Value}</c>.</p>
+
+    <p>In the expression at prompt <em>5&gt;</em> the expressions
+    inside the <seeguide marker="#block_expressions">block
+    expression</seeguide> are evaluated first, binding variable
+    <c>Y</c> and creating a binary.  The binary is then matched
+    against pattern <c>&lt;&lt;X:Y&gt;&gt;</c> using the value of
+    <c>Y</c> as the size of the segment.</p>
   </section>
 
   <section>
@@ -1767,6 +1776,7 @@ end</code>
   </section>
 
   <section>
+    <marker id="block_expressions"></marker>
     <title>Block Expressions</title>
     <pre>
 begin