added support for empty lists (aa[] = {};) and concatenation of lists
(aa[] += {1,2,3};)

Parser/Gmsh.y

 %{
-// $Id: Gmsh.y,v 1.219 2006-01-29 19:06:36 geuzaine Exp $
+// $Id: Gmsh.y,v 1.220 2006-02-17 14:35:06 geuzaine Exp $
 //
 // Copyright (C) 1997-2006 C. Geuzaine, J.-F. Remacle
 //
@@ -689,6 +689,24 @@ Affectation :
       }
       List_Delete($5);
     }
+  | tSTRING '[' ']' tAFFECTPLUS ListOfDouble tEND
+    {
+      // appends to the list
+      Symbol TheSymbol;
+      TheSymbol.Name = $1;
+      Symbol *pSymbol;
+      if(!(pSymbol = (Symbol*)Tree_PQuery(Symbol_T, &TheSymbol))){
+	TheSymbol.val = List_Create(5, 5, sizeof(double));
+	List_Copy($5, TheSymbol.val);
+	Tree_Add(Symbol_T, &TheSymbol);
+      }
+      else{
+	for(int i = 0; i < List_Nbr($5); i++)
+	  List_Add(pSymbol->val, List_Pointer($5, i));
+	Free($1);
+      }
+      List_Delete($5);
+    }
   | tSTRING NumericIncrement tEND
     {
       Symbol TheSymbol;
@@ -2890,6 +2908,11 @@ ListOfDouble :
     {
       $$ = $1;
     }
+  | '{' '}'
+    {
+      // creates an empty list
+      $$ = List_Create(2, 1, sizeof(double));
+    }
   | '{' RecursiveListOfDouble '}'
     {
       $$ = $2;

Parser/Gmsh.yy.cpp

@@ -2,7 +2,7 @@
 /* A lexical scanner generated by flex */
 
 /* Scanner skeleton version:
- * $Header: /cvsroot/gmsh/Parser/Gmsh.yy.cpp,v 1.254 2006-02-15 15:06:27 geuzaine Exp $
+ * $Header: /cvsroot/gmsh/Parser/Gmsh.yy.cpp,v 1.255 2006-02-17 14:35:06 geuzaine Exp $
  */
 
 #define FLEX_SCANNER
@@ -727,7 +727,7 @@ char *yytext;
 #line 1 "Gmsh.l"
 #define INITIAL 0
 #line 2 "Gmsh.l"
-// $Id: Gmsh.yy.cpp,v 1.254 2006-02-15 15:06:27 geuzaine Exp $
+// $Id: Gmsh.yy.cpp,v 1.255 2006-02-17 14:35:06 geuzaine Exp $
 //
 // Copyright (C) 1997-2006 C. Geuzaine, J.-F. Remacle
 //

benchmarks/misc/empty_list_append.geo

+// this is optional: you can actually append to a non-existent list:
+// it will create a new list
+aa[] = {};           Printf("aa size = %g", #aa[]);
+aa[] += 0;           Printf("aa size = %g", #aa[]);
+aa[] += {1,2,3};     Printf("aa size = %g", #aa[]);
+
+// this does not append: it adds the items in aa[] to the items
+// indexed by aa[]!
+aa[{aa[]}] += aa[];  Printf("aa size = %g", #aa[]);
+
+// this appends aa[] to itself
+aa[] += aa[];        Printf("aa size = %g", #aa[]);
+
+
+For i In {1 : #aa[]}
+  Printf("aa[%g] = %g", i-1, aa[i-1]);
+EndFor

doc/texinfo/gmsh.texi

 \input texinfo.tex @c -*-texinfo-*-
-@c $Id: gmsh.texi,v 1.199 2006-02-15 15:06:27 geuzaine Exp $
+@c $Id: gmsh.texi,v 1.200 2006-02-17 14:35:07 geuzaine Exp $
 @c
 @c Copyright (C) 1997-2006 C. Geuzaine, J.-F. Remacle
 @c
@@ -1140,7 +1140,7 @@ The following commands can be used anywhere in a Gmsh ASCII text input file:
 @ftable @code
 
 @item @var{string} = @var{expression};
-Defines a new expression identifier @var{string}, or affects
+Creates a new expression identifier @var{string}, or affects
 @var{expression} to an existing expression identifier. Eleven expression
 identifiers are predefined (hardcoded in Gmsh's parser):
 
@@ -1196,17 +1196,23 @@ of @code{newl}, @code{news}, @code{newv} and @code{newreg} can be modified
 with the @code{Geometry.OldNewReg} option (@pxref{Geometry options}).}.
 @end ftable
 
+@item @var{string} [ ] = @{ @};
+Creates a new expression list identifier @code{@var{string}[]} with an
+empty list.
+
 @item @var{string} [ ] = @{ @var{expression-list} @};
-Defines a new expression list identifier @code{@var{string}[]}, or affects
-@var{expression-list} to an existing expression list identifier.
+Creates a new expression list identifier @code{@var{string}[]} with the
+list @var{expression-list}, or affects @var{expression-list} to an
+existing expression list identifier. (Remember the remark we made when
+we defined @w{@var{expression-list}s}: the braces enclosing an
+@var{expression-list} are optional if the list only contains a single
+item.)
 
 @item @var{string} [ @{ @var{expression-list} @} ] = @{ @var{expression-list} @};
 Affects each item in the right hand side @var{expression-list} to the
 elements (indexed by the left hand side @var{expression-list}) of an
 existing expression list identifier. The two @w{@var{expression-list}s} must
-contain the same number of items. Remember the remark made when defining
-@w{@var{expression-list}s}: the braces enclosing an @var{expression-list}
-are optional if the list only contains a single item.
+contain the same number of items. 
 
 @item @var{real-option} = @var{expression};
 Affects @var{expression} to a real option.
@@ -1233,6 +1239,10 @@ or to a real option.
 Divides and affects @var{expression} to an existing expression identifier
 or to a real option.
 
+@item @var{string} [ ] += @{ @var{expression-list} @};
+Appends @var{expression-list} to an existing expression list or creates
+a new expression list with @var{expression-list}).
+
 @item @var{string} [ @{ @var{expression-list} @} ] += @{ @var{expression-list} @};
 Adds and affects, item per item, the right hand side @var{expression-list}
 to an existing expression list identifier.