add a note clarifying ceylon/ceylon.ast#118

typechecker/en/modules/expressions.xml

@@ -2631,6 +2631,12 @@ Digit{1,2} ":" Digit{2} ( ":" Digit{2} ( ":" Digit{3} )? )?
     <para>Operands within brackets in any subrange operator are parsed as if they
     were operands of the <literal>..</literal> or <literal>:</literal> operators.</para>
 
+    <comment><para>Note: an ambiguity exists in interpretation of expressions like
+    <literal>map[n..m]</literal> and <literal>map[0:l]</literal>, where the expression 
+    could in principle be interpreted as a lookup operator applied to a range 
+    constructor. This ambiguity is always resolved in favor or interpreting the 
+    expression as a subrange operator.</para></comment>
+
     <para>There are two special cases related to sequences. A type <literal>X</literal>
     is a <emphasis>sequence type</emphasis> if <literal>X</literal> is a subtype of 
     <literal>Sequential&lt;Anything&gt;</literal>.</para>