Generalize bit string comprehensions

The expression in a bit string comprehension is limited to a
literal bit string expression. That is, the following code
is legal:

  << <<X>> || X <- List >>

but not this code:

  << foo(X) || X <- List >>

The limitation is annoying. For one thing, tools that transform
the abstract format must be careful not to produce code such as:

  << begin
      %% Some instrumentation code.
      <<X>>
     end || X <- List >>

One reason for the limitation could be that we'll get
reduce/reduce conflicts if we try to allow an arbitrary
expression in a bit string comprehension:

  binary_comprehension -> '<<' expr '||' lc_exprs '>>' :
	{bc,?anno('$1'),'$2','$4'}.

Unfortunately, there does not seem to be an easy way to work
around that problem. The best we can do is to allow 'expr_max'
expressions (as in the binary syntax):

  binary_comprehension -> '<<' expr_max '||' lc_exprs '>>' :
	{bc,?anno('$1'),'$2','$4'}.

That will work, but functions calls must be enclosed in
parentheses:

  << (foo(X)) || X <- List >>

1 files changed, 5 insertions, 3 deletions

diff --git a/system/doc/reference_manual/expressions.xml b/system/doc/reference_manual/expressions.xml
index e98fcbcbb9..75486488e9 100644
--- a/system/doc/reference_manual/expressions.xml
+++ b/system/doc/reference_manual/expressions.xml
@@ -1556,9 +1556,11 @@ end</pre>
     <p>Bit string comprehensions are written with
     the following syntax:</p>
     <pre>
-&lt;&lt; BitString || Qualifier1,...,QualifierN &gt;&gt;</pre>
-    <p>Here, <c>BitString</c> is a bit string expression and each
-      <c>Qualifier</c> is either a generator, a bit string generator or a filter.</p>
+&lt;&lt; BitStringExpr || Qualifier1,...,QualifierN &gt;&gt;</pre>
+    <p><c>BitStringExpr</c> is an expression that evalutes to a bit
+    string. If <c>BitStringExpr</c> is a function call, it must be
+    enclosed in parentheses.  Each <c>Qualifier</c> is either a
+    generator, a bit string generator or a filter.</p>
     <list type="bulleted">
  <item>A <em>generator</em> is written as:      <br></br>
       &nbsp;&nbsp;<c><![CDATA[Pattern <- ListExpr]]></c>.      <br></br>