diff --git a/src/www/gfse/cloud.js b/src/www/gfse/cloud.js
index f19b6ee37..18cbaae9f 100644
--- a/src/www/gfse/cloud.js
+++ b/src/www/gfse/cloud.js
@@ -24,7 +24,7 @@ function upload(g) {
 		      [hidden(g.basename+".gf",show_abstract(g))])
 	for(var i in g.concretes)
 	    form.appendChild(hidden(g.basename+g.concretes[i].langcode+".gf",
-				    show_concrete(g.basename)(g.concretes[i])));
+				    show_concrete(g)(g.concretes[i])));
 	editor.appendChild(form);
 	form.submit();
 	form.parentNode.removeChild(form);
diff --git a/src/www/gfse/cloud2.js b/src/www/gfse/cloud2.js
index 8a6f198f3..4794e4ea0 100644
--- a/src/www/gfse/cloud2.js
+++ b/src/www/gfse/cloud2.js
@@ -36,7 +36,7 @@ function upload(g) {
 	    var cname=g.basename+g.concretes[i].langcode+".gf";
 	    //files.push(cname);
 	    form.appendChild(hidden(cname,
-				    show_concrete(g.basename)(g.concretes[i])));
+				    show_concrete(g)(g.concretes[i])));
 	}
 	editor.appendChild(form);
 	form.submit();
diff --git a/src/www/gfse/editor.css b/src/www/gfse/editor.css
index ec92a8e01..302548371 100644
--- a/src/www/gfse/editor.css
+++ b/src/www/gfse/editor.css
@@ -17,6 +17,9 @@ div#file, pre.plain { background: white; padding: 0.6ex; }
 img.cloud, img.right, div.right, div.modtime { float: right; }
 .modtime { color: #999; white-space: nowrap; }
 
+table.grammar_list { border-collapse: collapse; margin-left: 1.0em; }
+table.grammar_list td { padding: 0.4ex 0.25em; }
+
 div.namebar { background: #9df; }
 div.namebar table { width: 100%; }
 .namebar h3, .home h3 { margin: 0; color: #009; }
@@ -45,7 +48,8 @@ div.template:hover
 
 .editable:hover, .deletable:hover { background: #ff9; }
 
-.extensible:hover .more,.editable:hover > .edit ,.deletable:hover > .delete 
+.extensible:hover .more,.editable:hover > .edit ,.deletable:hover > .delete,
+ tr.deletable:hover .delete 
   { visibility: visible; }
 
 .more { color: green; }
diff --git a/src/www/gfse/editor.js b/src/www/gfse/editor.js
index 7f2112b6b..d2c41f30f 100644
--- a/src/www/gfse/editor.js
+++ b/src/www/gfse/editor.js
@@ -35,24 +35,29 @@ function draw_grammar_list() {
 	insertAfter(cloud_download,cloud_upload);
     }
     editor.appendChild(home)
-    var gs=ul([]);
     function del(i) { return function () { delete_grammar(i); } }
     function clone(i) { return function (g,b) { clone_grammar(i); } }
-    for(var i=0;i<local.count;i++) {
-	var grammar=local.get(i,null);
-	if(grammar && grammar.basename) {
-	    var link=a(jsurl("open_grammar("+i+")"),[text(grammar.basename)]);
-	    gs.appendChild(
-		node("li",{"class":"extensible"},
-		     [deletable(del(i),link,"Delete this grammar"),
-		      more(grammar,clone(i),"Clone this grammar")]))
-	}
+    function new_extension(i) { return function (g,b) { new_grammar([g]) }}
+    function item(i,grammar) {
+	var link=a(jsurl("open_grammar("+i+")"),[text(grammar.basename)]);
+	return node("tr",{"class":"extensible deletable"},
+		    [td(delete_button(del(i),"Delete this grammar")),
+		     td(link),
+		     td(more(grammar,clone(i),"Clone this grammar")),
+		     td(more(grammar,new_extension(i),"Create an extension of this grammar"))])
     }
     if(local.get("count",null)==null)
 	home.appendChild(text("You have not created any grammars yet."));
     else if(local.count==0)
 	home.appendChild(text("Your grammar list is empty."));
-    home.appendChild(gs);
+    else {
+	var rows=[];
+	for(var i=0;i<local.count;i++) {
+	    var grammar=local.get(i,null);
+	    if(grammar && grammar.basename) rows.push(item(i,grammar))
+	}
+	home.appendChild(node("table",{"class":"grammar_list"},rows));
+    }
 
     home.appendChild(
 	ul([li([a(jsurl("new_grammar()"),[text("New grammar")])])]));
@@ -60,13 +65,31 @@ function draw_grammar_list() {
     home.appendChild(empty_id("div","sharing"));
 }
 
-function new_grammar() {
+function new_grammar(gs) {
+    gs=gs || [];
     var g={basename:"Unnamed",
+	   extends:gs.map(function(g){return g.basename}),
+	             // check that "Unnamed" is not in exts !!
 	   abstract:{cats:[],funs:[]},
-	   concretes:[]}
+	   concretes:empty_concretes_extending(gs)}
     edit_grammar(g);
 }
 
+function empty_concretes_extending(gs) {
+    var concs=[]
+    var langs=[];
+    for(var i in gs) {
+	var g=gs[i];
+	for(var ci in g.concretes) {
+	    var conc=g.concretes[ci];
+	    var code=conc.langcode;
+	    if(!langs[code])
+		langs[code]=true,concs.push(new_concrete(code))
+	}
+    }
+    return concs
+}
+
 function remove_local_grammar(i) {
     local.remove(i);
     while(local.count>0 && !local.get(local.count-1))
@@ -291,6 +314,14 @@ function draw_startcat(g) {
     return indent([kw("flags startcat"),sep(" = "),m]);
 }
 
+function draw_extends(exts) {
+    var kw_extends=kw("extends ")
+    kw_extends.title="This grammar is an extension of the grammars listed here."
+    return exts && exts.length>0
+	? indent([kw_extends,ident(exts.join(", "))])
+	: text("")
+}
+
 function draw_abstract(g) {
     var kw_cat = kw("cat");
     kw_cat.title = "The categories (nonterminals) of the grammar are enumerated here. [C.3.2]";
@@ -307,6 +338,7 @@ function draw_abstract(g) {
     return div_id("file",
 		  [kw("abstract "),ident(g.basename),sep(" = "),
 		   draw_timestamp(g.abstract),
+		   draw_extends(g.extends),
 		   flags,
 		   indent([extensible([kw_cat,
 				       indent(draw_cats(g))]),
@@ -352,21 +384,27 @@ function rename_cat(g,el,cat) {
     string_editor(el,cat,ren);
 }
 
+function duplicated(g,kind,orig) {
+    return orig==g.basename
+	? "Same "+kind+" defined twice in this module"
+	: "Same "+kind+" already defined in "+orig
+}
+
 function draw_cats(g) {
     var cs=g.abstract.cats;
     var es=[];
-    var defined={};
+    var defined=inherited_cats(g);
     function eident(cat) {
 	function ren(g,el) { rename_cat(g,el,cat); }
 	return editable("span",ident(cat),g,ren,"Rename category");
     }
     function check(cat,el) {
-	return ifError(defined[cat],"Same category named twice",el);
+	return ifError(defined[cat],duplicated(g,"category",defined[cat]),el);
     }
     function del(i) { return function() { delete_cat(g,i); }}
     for(var i in cs) {
 	es.push(deletable(del(i),check(cs[i],eident(cs[i])),"Delete this category"));
-	defined[cs[i]]=true;
+	defined[cs[i]]=g.basename;
 	es.push(sep("; "));
     }
     es.push(more(g,add_cat,"Add more categories"));
@@ -419,24 +457,24 @@ function draw_funs(g) {
     var funs=g.abstract.funs;
     var es=[];
     var dc=defined_cats(g);
-    var df={};
+    var df=inherited_funs(g);
     function del(i) { return function() { delete_fun(g,i); }}
     function draw_efun(i,df) {
-	return editable("span",draw_fun(funs[i],dc,df),g,edit_fun(i),"Edit this function");
+	return editable("span",draw_fun(g,funs[i],dc,df),g,edit_fun(i),"Edit this function");
     }
     for(var i in funs) {
 	es.push(node_sortable("fun",funs[i].name,[deletable(del(i),draw_efun(i,df),"Delete this function")]));
-	df[funs[i].name]=true;
+	df[funs[i].name]=g.basename;
     }
     es.push(more(g,add_fun,"Add a new function"));
     return es;
 }
 
-function draw_fun(fun,dc,df) {
+function draw_fun(g,fun,dc,df) {
     function check(el) {
 	return ifError(dc[fun.name],
 		       "Function names must be distinct from category names",
-		       ifError(df[fun.name],"Same function defined twice",el));
+		       ifError(df[fun.name],duplicated(g,"function",df[fun.name]),el));
     }
     return node("span",{},
 		[check(ident(fun.name)),sep(" : "),draw_type(fun.type,dc)]);
@@ -496,6 +534,7 @@ function draw_concrete(g,i) {
 			    edit_langcode,"Change language"),
 		   kw(" of "),ident(g.basename),sep(" = "),
 		   draw_timestamp(conc),
+		   draw_extends((g.extends || []).map(conc_extends(conc))),
 		   indent([extensible([kw("open "),draw_opens(g,i)])]),
 		   indent([kw_lincat,draw_lincats(g,i)]),
 		   indent([kw_lin,draw_lins(g,i)]),
@@ -658,7 +697,7 @@ function draw_lincats(g,i) {
 	return node("span",{"class":cls},
 		    [ident(c.cat),sep(" = "),t]);
     }
-    var dc=defined_cats(g);
+    var dc=locally_defined_cats(g,{});
     function draw_lincat(c) {
 	var cat=c.cat;
 	var err=!dc[cat];
@@ -821,7 +860,7 @@ function draw_lins(g,ci) {
 	l.push(t);
 	return node("span",{"class":cls},l);
     }
-    var df=defined_funs(g);
+    var df=locally_defined_funs(g,{});
     function draw_lin(f) {
 	var fun=f.fun;
 	var err= !df[fun];
@@ -852,6 +891,42 @@ function draw_lins(g,ci) {
 
 /* -------------------------------------------------------------------------- */
 
+function defined_cats(g) {
+    var grammar_byname=cached_grammar_byname();
+    var igs=(g.extends || []).map(grammar_byname)
+    return all_defined_cats(g,igs)
+}
+
+function inherited_cats(g) {
+    var grammar_byname=cached_grammar_byname();
+    var igs=(g.extends || []).map(grammar_byname)
+    return all_inherited_cats(igs,{})
+}
+
+function defined_funs(g) {
+    var grammar_byname=cached_grammar_byname();
+    var igs=(g.extends || []).map(grammar_byname)
+    return all_defined_funs(g,igs)
+}
+
+function inherited_funs(g) {
+    var grammar_byname=cached_grammar_byname();
+    var igs=(g.extends || []).map(grammar_byname)
+    return all_inherited_funs(igs,{})
+}
+
+function cached_grammar_byname() {
+    var gix={};
+    for(var i=0;i<local.count;i++) {
+	var g=local.get(i,null);
+	if(g) gix[g.basename]=g; // basenames are not necessarily unique!!
+    }
+    function grammar_byname(name) { return gix[name]; }
+    return grammar_byname;
+}
+
+/* -------------------------------------------------------------------------- */
+
 function find_langcode(concs,langcode) {
     for(var ci in concs)
 	if(concs[ci].langcode==langcode)
@@ -1030,11 +1105,16 @@ function edit_button(action,hint) {
 }
 
 function deletable(del,el,hint) {
-    var b=node("span",{"class":"delete",title:hint || "Delete"},[text("×")])
-    b.onclick=del;
+    var b=delete_button(del,hint)
     return node("span",{"class":"deletable"},[b,el])
 }
 
+function delete_button(action,hint) {
+    var b=node("span",{"class":"delete",title:hint || "Delete"},[text("×")])
+    b.onclick=action;
+    return b;
+}
+
 function touch_edit() {
     var b=node("input",{type:"checkbox"},[]);
     function touch() {
diff --git a/src/www/gfse/gf_abs.js b/src/www/gfse/gf_abs.js
index 953a69683..e4829548a 100644
--- a/src/www/gfse/gf_abs.js
+++ b/src/www/gfse/gf_abs.js
@@ -2,20 +2,25 @@
 
 /*
 type Id   = String -- all sorts of identifiers
+type ModId = Id -- module name
 type Cat  = Id -- category name
+type FunId = Id -- function name
 type Type = [Cat] -- [Cat_1,...,Cat_n] means Cat_1 -> ... -> Cat_n
 
-type Grammar  = { basename: Id, abstract : Abstract, concretes : [Concrete] }
+type Grammar  = { basename: ModId,
+                  extends: [ModId],
+                  abstract: Abstract,
+                  concretes: [Concrete] }
 
-type Abstract = { startcat: Cat, cats : [Cat], funs : [ Fun ] }
-type Fun      = { name:Id, type : Type }
+type Abstract = { startcat: Cat, cats: [Cat], funs: [Fun] }
+type Fun      = { name: FunId, type: Type }
 
-type Concrete = { langcode:Id,
-                  opens:[Id],
-		  params: [{name:Id, rhs:String}],
-		  lincats : [{ cat:Cat, type:Term}],
-		  opers: [{name:Lhs, rhs:Term}],
-		  lins: [{fun:Id, args:[Id], lin:Term}]
+type Concrete = { langcode: Id,
+                  opens: [ModId],
+		  params: [{name: Id, rhs: String}],
+		  lincats : [{ cat: Cat, type: Term}],
+		  opers: [{name: Lhs, rhs: Term}],
+		  lins: [{fun: FunId, args: [Id], lin: Term}]
 		}
 
 
@@ -24,24 +29,42 @@ type Lhs = String -- name and type of oper,
 type Term = String -- arbitrary GF term (not parsed by the editor)
 */
 
-// defined_cats :: [Grammar] -> {Cat=>Bool}
-function defined_cats(g) {
-    var dc={};
+// locally_defined_cats :: Grammar -> {Cat=>Bool} -> {Cat=>Bool} // destr upd
+function locally_defined_cats(g,dc) {
     with(g.abstract)
-	for(var i in cats) dc[cats[i]]=true;
+	for(var i in cats) dc[cats[i]]=g.basename;
     return dc;
 }
 
-// defined_funs :: [Grammar] -> {Id=>Bool}
-function defined_funs(g) {
-    var df={};
+// all_defined_cats :: Grammar -> [Grammar] -> {Cat=>Bool}
+function all_defined_cats(g,igs) {
+    return all_inherited_cats(igs,locally_defined_cats(g,{}))
+}
+// all_inherited_cats :: [Grammar] -> {Cat=>Bool} -> {Cat=>Bool} // destr upd
+function all_inherited_cats(igs,dc) {
+    for(var i in igs) dc=locally_defined_cats(igs[i],dc)
+    return dc;
+}
+
+// locally_defined_funs :: [Grammar] -> {FunId=>ModId} -> {Id=>ModId} // destr upd
+function locally_defined_funs(g,df) {
     with(g.abstract)
-	for(var i in funs) df[funs[i].name]=true;
+	for(var i in funs) df[funs[i].name]=g.basename;
+    return df;
+}
+
+// all_defined_funs :: Grammar -> [Grammar] -> {FunId=>ModId}
+function all_defined_funs(g,igs) {
+    return all_inherited_funs(igs,locally_defined_funs(g,{}))
+}
+// all_inherited_funs :: [Grammar] -> {FunId=>ModId} -> {FunId=>ModId} // destr upd
+function all_inherited_funs(igs,df) {
+    for(var i in igs) df=locally_defined_funs(igs[i],df)
     return df;
 }
 
 // Return the type of a named function in the abstract syntax
-// function_type :: Grammar -> Id -> Type
+// function_type :: Grammar -> FunId -> Type
 function function_type(g,fun) {
     with(g.abstract)
 	for(var i in funs) if(funs[i].name==fun) return funs[i].type
@@ -187,7 +210,7 @@ function parse_oper(s) {
     
 }
 
-/* --- Print as plain text (normal GF concrete syntax) ---------------------- */
+/* --- Print as plain text (normal GF source syntax) ------------------------- */
 
 function show_type(t) {
     var s="";
@@ -209,7 +232,9 @@ function show_grammar(g) {
 function show_abstract(g) {
 //  var startcat= g.abstract.cats.length==1 ? g.abstract.cats[0] : g.abstract.startcat;
     var startcat= g.abstract.startcat || g.abstract.cats[0];
-    return "abstract "+g.basename+" = {\n\n"
+    return "abstract "+g.basename+" = "
+        +show_extends(g.extends)
+        +"{\n\n"
 	+"flags coding = utf8 ;\n\n"
 	+show_startcat(startcat)
         +show_cats(g.abstract.cats)
@@ -217,6 +242,10 @@ function show_abstract(g) {
 	+"}\n";
 }
 
+function show_extends(exts) {
+    return exts && exts.length>0 ? exts.join(", ")+" ** " : "";
+}
+
 function show_startcat(startcat) {
     return startcat ? "flags startcat = "+startcat+";\n\n" : "";
 }
@@ -228,15 +257,18 @@ function show_cats(cats) {
 function show_funs(funs) { return show_list("fun",show_fun,funs); }
 
 function show_concretes(g) {
-    return map(show_concrete(g.basename),g.concretes).join("\n\n");
+    return map(show_concrete(g),g.concretes).join("\n\n");
 }
 
-function show_concrete(basename) {
+function conc_extends(conc) { return function(m) { return m+conc.langcode; }}
+
+function show_concrete(g) {
     return function(conc) {
 	return "--# -path=.:present\n"
-            + "concrete "+basename+conc.langcode+" of "+basename+" ="
+            + "concrete "+g.basename+conc.langcode+" of "+g.basename+" = "
+	    +show_extends((g.extends || []).map(conc_extends(conc)))
             +show_opens(conc.opens)
-	    +" {\n\nflags coding = utf8 ;\n\n"
+	    +"{\n\nflags coding = utf8 ;\n\n"
 	    +show_params(conc.params)
 	    +show_lincats(conc.lincats)
 	    +show_opers(conc.opers)
@@ -252,7 +284,7 @@ function show_list(kw,show1,list) {
 }
 
 function show_opens(opens) {
-    return opens && opens.length>0 ? "\n\nopen "+opens.join(", ")+" in" : ""
+    return opens && opens.length>0 ? "\n\nopen "+opens.join(", ")+" in " : ""
 }
 
 function show_params(params) { return show_list("param",show_param,params); }