diff --git a/src/editor/simple/about.html b/src/editor/simple/about.html
index 050812ca3..a488c6e56 100644
--- a/src/editor/simple/about.html
+++ b/src/editor/simple/about.html
@@ -116,7 +116,8 @@ At the moment, the concrete syntax for a language <var>L</var> is limited to
   <li><em>linearizations</em> for the functions in the abstract syntax,
   <li><em>parameter type definitions</em>,
   <var>P</var> = <var>C<sub>1</sub></var> | ... |<var>C<sub>n</sub></var>,
-  <li>and <em>operation definitions</em>, <var>op</var> = <var>expr</var>.
+  <li>and <em>operation definitions</em>, <var>op</var> = <var>expr</var>,
+   <var>op</var> : <var>type</var> = <var>expr</var>,
 </ul>
 
 Available editing operations:
@@ -146,8 +147,11 @@ Also,
 
 Error checks:
 <ul>
-  <li>The RHSs in the concrete syntax are not checked for errors. Arbitrary
-  strings can be entered.
+  <li>The RHSs in the concrete syntax are checked
+  for syntactic correctness by the editor as they are entered.
+  (TODO: the syntax of parameter types is not check at the moment.)
+  <li>Duplicated definitions are highlighted. Checks for other
+  semantic errors are delayed until the grammar is compiled.
 </ul>
 
 <h3>Compiling and testing grammars</h3>
@@ -164,7 +168,7 @@ cloud</h3>
 
 While the editor normally stores grammars locally in the browser, it is also
 possible to store grammars in the cloud.  Grammars can be stored in the cloud
-just for backup, or to accessed them from multiple devices.
+just for backup, or to make them accessible from multiple devices.
 
 <p>
 There is no automatic synchronization between local grammars and the cloud.
@@ -197,14 +201,13 @@ devices, but not recommended for sharing grammars between multiple users.
 
 <p>
 Also <strong>note</strong> that each grammar is assigned a unique identity
-when it is created. Renaming a grammar does not change its identity. This means
-that name changes are propagated between devices like other changes.
+when it is first created. Renaming a grammar does not change its identity.
+This means that name changes are propagated between devices like other changes.
 
 <h3>Future work</h3>
 
 This prototype gives an idea of how a web based GF grammar editor could work.
-While this editor is implemented in JavaScript and runs entirely in the
-web browser,
+While this editor is implemented in JavaScript and runs in the web browser,
 we do not expect to create a full implementation of GF that runs in the
 web browser, but let the editor communicate with a server running GF.
 <p>
@@ -214,7 +217,7 @@ to do proper error checking and make more of the existing GF shell functionality
 accessible directly from the editor.
 <p>
 The current grammar cloud service is very primitive. In particular, it is not
-suitable for multiple users developing  a grammar in collaboration.
+suitable for multiple users developing a grammar in collaboration.
 
 <h3>Related documents</h3>
 <ul>
@@ -225,7 +228,7 @@ suitable for multiple users developing  a grammar in collaboration.
 
 <hr>
 <div class=modtime><small>
-<!-- hhmts start --> Last modified: Tue Jul 26 17:01:22 CEST 2011 <!-- hhmts end -->
+<!-- hhmts start --> Last modified: Fri Jul 29 17:44:00 CEST 2011 <!-- hhmts end -->
   </small></div>
 <address>
 <a href="http://www.cse.chalmers.se/~hallgren/">TH</a>
diff --git a/src/editor/simple/cloud.js b/src/editor/simple/cloud.js
index cd2b9a442..fad66e1ff 100644
--- a/src/editor/simple/cloud.js
+++ b/src/editor/simple/cloud.js
@@ -136,6 +136,11 @@ function gfshell(cmd,cont) {
 
 // Check the syntax of an expression
 function check_exp(s,cont) {
-    // Not implemented yet!!
-    cont(null)
+    function check(gf_message) {
+	debug("cc "+s+" = "+gf_message);
+	cont(/parse error/.test(gf_message) ? "parse error" : null);
+    }
+    if(navigator.onLine)
+	ajax_http_get("upload.cgi?cc="+encodeURIComponent(s),check)
+    else cont(null)
 }
diff --git a/src/editor/simple/editor.js b/src/editor/simple/editor.js
index bbd4b7c9b..ce6351367 100644
--- a/src/editor/simple/editor.js
+++ b/src/editor/simple/editor.js
@@ -671,39 +671,50 @@ function draw_oper(p,dp) {
     return node("span",{},[check(ident(p.name)),text(" "),text(p.rhs)]);
 }
 
+function check_oper(s,ok,err) {
+    var p=parse_oper(s);
+    function check2(msg) {
+	if(msg) err(msg);
+	else ok(p.ok)
+    }
+    if(p.ok) {
+	// Checking oper syntax by checking an expression with a local
+	// definition. Some valid opers will be rejected!!
+	var e=p.ok.name+" where { "+p.ok.name+" "+p.ok.rhs+" }";
+	check_exp(e,check2);
+    }
+    else
+	err(p.error);
+}
+
 function add_oper(g,ci,el) {
-    function add(s) {
-	var p=parse_oper(s);
-	if(p.ok) {
-	    g.concretes[ci].opers.push(p.ok);
+    function check(s,cont) {
+	function ok(oper) {
+	    g.concretes[ci].opers.push(oper);
 	    timestamp(g.concretes[ci]);
 	    reload_grammar(g);
-	    return null;
+	    cont(null);
 	}
-	else
-	    return p.error
+	check_oper(s,ok,cont)
     }
-    string_editor(el,"",add);
+    string_editor(el,"",check,true);
 }
 
 function edit_oper(ci,i) {
     return function (g,el) {
-	function replace(s) {
-	    var p=parse_oper(s);
-	    if(p.ok) {
-		g.concretes[ci].opers[i]=p.ok;
+	function check(s,cont) {
+	    function ok(oper) {
+		g.concretes[ci].opers[i]=oper;
 		timestamp(g.concretes[ci]);
 		reload_grammar(g);
-		return null;
+		cont(null);
 	    }
-	    else
-		return p.error;
+	    check_oper(s,ok,cont)
 	}
-	string_editor(el,show_oper(g.concretes[ci].opers[i]),replace);
+	string_editor(el,show_oper(g.concretes[ci].opers[i]),check,true);
     }
 }
 
-
 function delete_oper(g,ci,ix) {
     with(g.concretes[ci]) opers=delete_ix(opers,ix);
     timestamp(g.concretes[ci]);
diff --git a/src/editor/simple/upload.cgi b/src/editor/simple/upload.cgi
index 3da386dd6..662a1d35d 100644
--- a/src/editor/simple/upload.cgi
+++ b/src/editor/simple/upload.cgi
@@ -219,6 +219,13 @@ case "$REQUEST_METHOD" in
                 *) error400
 	     esac
 	     ;;
+	cc=*)
+	     # Just to check an expression for syntax errors
+	     exp=$(qparse "$QUERY_STRING" cc)
+	     ContentType="text/plain; charset=$charset"
+	     cgiheaders
+	     echo "cc $exp" | GF_RESTRICTED=True gf -run
+	     ;;
         *) error400
     esac
 esac