diff --git a/Demo/tkinter/guido/ss1.py b/Demo/tkinter/guido/ss1.py
new file mode 100644
index 0000000..ccbb9fc
--- /dev/null
+++ b/Demo/tkinter/guido/ss1.py
@@ -0,0 +1,740 @@
+"""SS1 -- a spreadsheet."""
+
+import os
+import re
+import sys
+import cgi
+import rexec
+from xml.parsers import expat
+
+LEFT, CENTER, RIGHT = "LEFT", "CENTER", "RIGHT"
+
+def ljust(x, n):
+    return x.ljust(n)
+def center(x, n):
+    return x.center(n)
+def rjust(x, n):
+    return x.rjust(n)
+align2action = {LEFT: ljust, CENTER: center, RIGHT: rjust}
+
+align2xml = {LEFT: "left", CENTER: "center", RIGHT: "right"}
+xml2align = {"left": LEFT, "center": CENTER, "right": RIGHT}
+
+align2anchor = {LEFT: "w", CENTER: "center", RIGHT: "e"}
+
+def sum(seq):
+    total = 0
+    for x in seq:
+        if x is not None:
+            total += x
+    return total
+
+class Sheet:
+
+    def __init__(self):
+        self.cells = {} # {(x, y): cell, ...}
+        self.rexec = rexec.RExec()
+        m = self.rexec.add_module('__main__')
+        m.cell = self.cellvalue
+        m.cells = self.multicellvalue
+        m.sum = sum
+
+    def cellvalue(self, x, y):
+        cell = self.getcell(x, y)
+        if hasattr(cell, 'recalc'):
+            return cell.recalc(self.rexec)
+        else:
+            return cell
+
+    def multicellvalue(self, x1, y1, x2, y2):
+        if x1 > x2:
+            x1, x2 = x2, x1
+        if y1 > y2:
+            y1, y2 = y2, y1
+        seq = []
+        for y in range(y1, y2+1):
+            for x in range(x1, x2+1):
+                seq.append(self.cellvalue(x, y))
+        return seq
+
+    def getcell(self, x, y):
+        return self.cells.get((x, y))
+
+    def setcell(self, x, y, cell):
+        assert x > 0 and y > 0
+        assert isinstance(cell, BaseCell)
+        self.cells[x, y] = cell
+
+    def clearcell(self, x, y):
+        try:
+            del self.cells[x, y]
+        except KeyError:
+            pass
+
+    def clearcells(self, x1, y1, x2, y2):
+        for xy in self.selectcells(x1, y1, x2, y2):
+            del self.cells[xy]
+
+    def clearrows(self, y1, y2):
+        self.clearcells(0, y1, sys.maxint, y2)
+
+    def clearcolumns(self, x1, x2):
+        self.clearcells(x1, 0, x2, sys.maxint)
+
+    def selectcells(self, x1, y1, x2, y2):
+        if x1 > x2:
+            x1, x2 = x2, x1
+        if y1 > y2:
+            y1, y2 = y2, y1
+        return [(x, y) for x, y in self.cells
+                if x1 <= x <= x2 and y1 <= y <= y2]
+
+    def movecells(self, x1, y1, x2, y2, dx, dy):
+        if dx == 0 and dy == 0:
+            return
+        if x1 > x2:
+            x1, x2 = x2, x1
+        if y1 > y2:
+            y1, y2 = y2, y1
+        assert x1+dx > 0 and y1+dy > 0
+        new = {}
+        for x, y in self.cells:
+            cell = self.cells[x, y]
+            if hasattr(cell, 'renumber'):
+                cell = cell.renumber(x1, y1, x2, y2, dx, dy)
+            if x1 <= x <= x2 and y1 <= y <= y2:
+                x += dx
+                y += dy
+            new[x, y] = cell
+        self.cells = new
+
+    def insertrows(self, y, n):
+        assert n > 0
+        self.movecells(0, y, sys.maxint, sys.maxint, 0, n)
+
+    def deleterows(self, y1, y2):
+        if y1 > y2:
+            y1, y2 = y2, y1
+        self.clearrows(y1, y2)
+        self.movecells(0, y2+1, sys.maxint, sys.maxint, 0, y1-y2-1)
+
+    def insertcolumns(self, x, n):
+        assert n > 0
+        self.movecells(x, 0, sys.maxint, sys.maxint, n, 0)
+
+    def deletecolumns(self, x1, x2):
+        if x1 > x2:
+            x1, x2 = x2, x1
+        self.clearcells(x1, x2)
+        self.movecells(x2+1, 0, sys.maxint, sys.maxint, x1-x2-1, 0)
+
+    def getsize(self):
+        maxx = maxy = 0
+        for x, y in self.cells:
+            maxx = max(maxx, x)
+            maxy = max(maxy, y)
+        return maxx, maxy
+
+    def reset(self):
+        for cell in self.cells.itervalues():
+            if hasattr(cell, 'reset'):
+                cell.reset()
+
+    def recalc(self):
+        self.reset()
+        for cell in self.cells.itervalues():
+            if hasattr(cell, 'recalc'):
+                cell.recalc(self.rexec)
+
+    def display(self):
+        maxx, maxy = self.getsize()
+        width, height = maxx+1, maxy+1
+        colwidth = [1] * width
+        full = {}
+        # Add column heading labels in row 0
+        for x in range(1, width):
+            full[x, 0] = text, alignment = colnum2name(x), RIGHT
+            colwidth[x] = max(colwidth[x], len(text))
+        # Add row labels in column 0
+        for y in range(1, height):
+            full[0, y] = text, alignment = str(y), RIGHT
+            colwidth[0] = max(colwidth[0], len(text))
+        # Add sheet cells in columns with x>0 and y>0
+        for (x, y), cell in self.cells.iteritems():
+            if x <= 0 or y <= 0:
+                continue
+            if hasattr(cell, 'recalc'):
+                cell.recalc(self.rexec)
+            if hasattr(cell, 'format'):
+                text, alignment = cell.format()
+                assert isinstance(text, str)
+                assert alignment in (LEFT, CENTER, RIGHT)
+            else:
+                text = str(cell)
+                if isinstance(cell, str):
+                    alignment = LEFT
+                else:
+                    alignment = RIGHT
+            full[x, y] = (text, alignment)
+            colwidth[x] = max(colwidth[x], len(text))
+        # Calculate the horizontal separator line (dashes and dots)
+        sep = ""
+        for x in range(width):
+            if sep:
+                sep += "+"
+            sep += "-"*colwidth[x]
+        # Now print The full grid
+        for y in range(height):
+            line = ""
+            for x in range(width):
+                text, alignment = full.get((x, y)) or ("", LEFT)
+                text = align2action[alignment](text, colwidth[x])
+                if line:
+                    line += '|'
+                line += text
+            print line
+            if y == 0:
+                print sep
+
+    def xml(self):
+        out = ['<spreadsheet>']
+        for (x, y), cell in self.cells.iteritems():
+            if hasattr(cell, 'xml'):
+                cellxml = cell.xml()
+            else:
+                cellxml = '<value>%s</value>' % cgi.escape(cell)
+            out.append('<cell row="%s" col="%s">\n  %s\n</cell>' %
+                       (y, x, cellxml))
+        out.append('</spreadsheet>')
+        return '\n'.join(out)
+
+    def save(self, filename):
+        text = self.xml()
+        f = open(filename, "w")
+        f.write(text)
+        if text and not text.endswith('\n'):
+            f.write('\n')
+        f.close()
+
+    def load(self, filename):
+        f = open(filename, 'r')
+        SheetParser(self).parsefile(f)
+        f.close()
+
+class SheetParser:
+
+    def __init__(self, sheet):
+        self.sheet = sheet
+
+    def parsefile(self, f):
+        parser = expat.ParserCreate()
+        parser.StartElementHandler = self.startelement
+        parser.EndElementHandler = self.endelement
+        parser.CharacterDataHandler = self.data
+        parser.ParseFile(f)
+
+    def startelement(self, tag, attrs):
+        method = getattr(self, 'start_'+tag, None)
+        if method:
+            for key, value in attrs.iteritems():
+                attrs[key] = str(value) # XXX Convert Unicode to 8-bit
+            method(attrs)
+        self.texts = []
+
+    def data(self, text):
+        text = str(text) # XXX Convert Unicode to 8-bit
+        self.texts.append(text)
+
+    def endelement(self, tag):
+        method = getattr(self, 'end_'+tag, None)
+        if method:
+            method("".join(self.texts))
+
+    def start_cell(self, attrs):
+        self.y = int(attrs.get("row"))
+        self.x = int(attrs.get("col"))
+
+    def start_value(self, attrs):
+        self.fmt = attrs.get('format')
+        self.alignment = xml2align.get(attrs.get('align'))
+
+    start_formula = start_value
+
+    def end_int(self, text):
+        try:
+            self.value = int(text)
+        except:
+            self.value = None
+
+    def end_long(self, text):
+        try:
+            self.value = long(text)
+        except:
+            self.value = None
+
+    def end_double(self, text):
+        try:
+            self.value = float(text)
+        except:
+            self.value = None
+
+    def end_complex(self, text):
+        try:
+            self.value = complex(text)
+        except:
+            self.value = None
+
+    def end_string(self, text):
+        try:
+            self.value = text
+        except:
+            self.value = None
+
+    def end_value(self, text):
+        if isinstance(self.value, BaseCell):
+            self.cell = self.value
+        elif isinstance(self.value, str):
+            self.cell = StringCell(self.value,
+                                   self.fmt or "%s",
+                                   self.alignment or LEFT)
+        else:
+            self.cell = NumericCell(self.value,
+                                    self.fmt or "%s",
+                                    self.alignment or RIGHT)
+
+    def end_formula(self, text):
+        self.cell = FormulaCell(text,
+                                self.fmt or "%s",
+                                self.alignment or RIGHT)
+
+    def end_cell(self, text):
+        self.sheet.setcell(self.x, self.y, self.cell)
+
+class BaseCell:
+    __init__ = None # Must provide
+    """Abstract base class for sheet cells.
+
+    Subclasses may but needn't provide the following APIs:
+    
+    cell.reset() -- prepare for recalculation
+    cell.recalc(rexec) -> value -- recalculate formula
+    cell.format() -> (value, alignment) -- return formatted value
+    cell.xml() -> string -- return XML
+    """
+
+class NumericCell(BaseCell):
+
+    def __init__(self, value, fmt="%s", alignment=RIGHT):
+        assert isinstance(value, (int, long, float, complex))
+        assert alignment in (LEFT, CENTER, RIGHT)
+        self.value = value
+        self.fmt = fmt
+        self.alignment = alignment
+
+    def recalc(self, rexec):
+        return self.value
+
+    def format(self):
+        try:
+            text = self.fmt % self.value
+        except:
+            text = str(self.value)
+        return text, self.alignment
+
+    def xml(self):
+        method = getattr(self, '_xml_' + type(self.value).__name__)
+        return '<value align="%s" format="%s">%s</value>' % (
+                align2xml[self.alignment],
+                self.fmt,
+                method())
+
+    def _xml_int(self):
+        if -2**31 <= self.value < 2**31:
+            return '<int>%s</int>' % self.value
+        else:
+            return self._xml_long()
+
+    def _xml_long(self):
+        return '<long>%s</long>' % self.value
+
+    def _xml_float(self):
+        return '<double>%s</double>' % repr(self.value)
+
+    def _xml_complex(self):
+        return '<complex>%s</double>' % repr(self.value)
+
+class StringCell(BaseCell):
+
+    def __init__(self, text, fmt="%s", alignment=LEFT):
+        assert isinstance(text, (str, unicode))
+        assert alignment in (LEFT, CENTER, RIGHT)
+        self.text = text
+        self.fmt = fmt
+        self.alignment = alignment
+
+    def recalc(self, rexec):
+        return self.text
+
+    def format(self):
+        return self.text, self.alignment
+
+    def xml(self):
+        s = '<value align="%s" format="%s"><string>%s</string></value>'
+        return s % (
+            align2xml[self.alignment],
+            self.fmt,
+            cgi.escape(self.text))
+
+class FormulaCell(BaseCell):
+
+    def __init__(self, formula, fmt="%s", alignment=RIGHT):
+        assert alignment in (LEFT, CENTER, RIGHT)
+        self.formula = formula
+        self.translated = translate(self.formula)
+        self.fmt = fmt
+        self.alignment = alignment
+        self.reset()
+
+    def reset(self):
+        self.value = None
+
+    def recalc(self, rexec):
+        if self.value is None:
+            try:
+                self.value = rexec.r_eval(self.translated)
+            except:
+                exc = sys.exc_info()[0]
+                if hasattr(exc, "__name__"):
+                    self.value = exc.__name__
+                else:
+                    self.value = str(exc)
+        return self.value
+
+    def format(self):
+        try:
+            text = self.fmt % self.value
+        except:
+            text = str(self.value)
+        return text, self.alignment
+
+    def xml(self):
+        return '<formula align="%s" format="%s">%s</formula>' % (
+            align2xml[self.alignment],
+            self.fmt,
+            self.formula)
+
+    def renumber(self, x1, y1, x2, y2, dx, dy):
+        out = []
+        for part in re.split('(\w+)', self.formula):
+            m = re.match('^([A-Z]+)([1-9][0-9]*)$', part)
+            if m is not None:
+                sx, sy = m.groups()
+                x = colname2num(sx)
+                y = int(sy)
+                if x1 <= x <= x2 and y1 <= y <= y2:
+                    part = cellname(x+dx, y+dy)
+            out.append(part)
+        return FormulaCell("".join(out), self.fmt, self.alignment)
+
+def translate(formula):
+    """Translate a formula containing fancy cell names to valid Python code.
+
+    Examples:
+        B4 -> cell(2, 4)
+        B4:Z100 -> cells(2, 4, 26, 100)
+    """
+    out = []
+    for part in re.split(r"(\w+(?::\w+)?)", formula):
+        m = re.match(r"^([A-Z]+)([1-9][0-9]*)(?::([A-Z]+)([1-9][0-9]*))?$", part)
+        if m is None:
+            out.append(part)
+        else:
+            x1, y1, x2, y2 = m.groups()
+            x1 = colname2num(x1)
+            if x2 is None:
+                s = "cell(%s, %s)" % (x1, y1)
+            else:
+                x2 = colname2num(x2)
+                s = "cells(%s, %s, %s, %s)" % (x1, y1, x2, y2)
+            out.append(s)
+    return "".join(out)
+
+def cellname(x, y):
+    "Translate a cell coordinate to a fancy cell name (e.g. (1, 1)->'A1')."
+    assert x > 0 # Column 0 has an empty name, so can't use that
+    return colnum2name(x) + str(y)
+
+def colname2num(s):
+    "Translate a column name to number (e.g. 'A'->1, 'Z'->26, 'AA'->27)."
+    s = s.upper()
+    n = 0
+    for c in s:
+        assert 'A' <= c <= 'Z'
+        n = n*26 + ord(c) - ord('A') + 1
+    return n
+
+def colnum2name(n):
+    "Translate a column number to name (e.g. 1->'A', etc.)."
+    assert n > 0
+    s = ""
+    while n:
+        n, m = divmod(n-1, 26)
+        s = chr(m+ord('A')) + s
+    return s
+
+import Tkinter as Tk
+
+class SheetGUI:
+
+    """Beginnings of a GUI for a spreadsheet.
+
+    TO DO:
+    - clear multiple cells
+    - Select rows or columns
+    - Insert, clear, remove rows or columns
+    - Show new contents while typing
+    - Scroll bars
+    - Grow grid when window is grown
+    - Proper menus
+    - Undo, redo
+    - Cut, copy and paste
+    - Formatting and alignment
+    """
+
+    def __init__(self, filename="sheet1.xml", rows=10, columns=5):
+        """Constructor.
+
+        Load the sheet from the filename argument.
+        Set up the Tk widget tree.
+        """
+        # Create and load the sheet
+        self.filename = filename
+        self.sheet = Sheet()
+        if os.path.isfile(filename):
+            self.sheet.load(filename)
+        # Calculate the needed grid size
+        maxx, maxy = self.sheet.getsize()
+        rows = max(rows, maxy)
+        columns = max(columns, maxx)
+        # Create the widgets
+        self.root = Tk.Tk()
+        self.root.wm_title("Spreadsheet: %s" % self.filename)
+        self.beacon = Tk.Label(self.root, text="A1",
+                               font=('helvetica', 16, 'bold'))
+        self.entry = Tk.Entry(self.root)
+        self.savebutton = Tk.Button(self.root, text="Save",
+                                    command=self.save)
+        self.cellgrid = Tk.Frame(self.root)
+        # Configure the widget lay-out
+        self.cellgrid.pack(side="bottom", expand=1, fill="both")
+        self.beacon.pack(side="left")
+        self.savebutton.pack(side="right")
+        self.entry.pack(side="left", expand=1, fill="x")
+        # Bind some events
+        self.entry.bind("<Return>", self.return_event)
+        self.entry.bind("<Shift-Return>", self.shift_return_event)
+        self.entry.bind("<Tab>", self.tab_event)
+        self.entry.bind("<Shift-Tab>", self.shift_tab_event)
+        # Now create the cell grid
+        self.makegrid(rows, columns)
+        # Select the top-left cell
+        self.currentxy = None
+        self.cornerxy = None
+        self.setcurrent(1, 1)
+        # Copy the sheet cells to the GUI cells
+        self.sync()
+
+    def makegrid(self, rows, columns):
+        """Helper to create the grid of GUI cells.
+
+        The edge (x==0 or y==0) is filled with labels; the rest is real cells.
+        """
+        self.gridcells = {}
+        # Create the top row of labels
+        for x in range(1, columns+1):
+            self.cellgrid.grid_columnconfigure(x, minsize=64)
+            cell = Tk.Label(self.cellgrid, text=colnum2name(x), relief='raised')
+            cell.grid_configure(column=x, row=0, sticky='WE')
+            self.gridcells[x, 0] = cell
+        # Create the leftmost column of labels
+        for y in range(1, rows+1):
+            cell = Tk.Label(self.cellgrid, text=str(y), relief='raised')
+            cell.grid_configure(column=0, row=y, sticky='WE')
+            self.gridcells[0, y] = cell
+        # Create the real cells
+        for x in range(1, columns+1):
+            for y in range(1, rows+1):
+                cell = Tk.Label(self.cellgrid, relief='sunken',
+                                bg='white', fg='black')
+                cell.grid_configure(column=x, row=y, sticky='NWSE')
+                self.gridcells[x, y] = cell
+                def helper(event, self=self, x=x, y=y):
+                    self.setcurrent(x, y)
+                cell.bind("<Button-1>", helper)
+                def shelper(event, self=self, x=x, y=y):
+                    self.setcorner(x, y)
+                cell.bind("<Shift-Button-1>", shelper)
+
+    def save(self):
+        self.sheet.save(self.filename)
+
+    def setcurrent(self, x, y):
+        "Make (x, y) the current cell."
+        if self.currentxy is not None:
+            self.change_cell()
+        self.clearfocus()
+        name = cellname(x, y)
+        cell = self.sheet.getcell(x, y)
+        if cell is None:
+            text = ""
+        elif isinstance(cell, FormulaCell):
+            text = '=' + cell.formula
+        else:
+            text, alignment = cell.format()
+        self.beacon['text'] = name
+        self.entry.delete(0, 'end')
+        self.entry.insert(0, text)
+        self.entry.selection_range(0, 'end')
+        self.entry.focus_set()
+        self.currentxy = x, y
+        self.cornerxy = None
+        gridcell = self.gridcells.get(self.currentxy)
+        if gridcell is not None:
+            gridcell['bg'] = 'lightBlue'
+
+    def setcorner(self, x, y):
+        if self.currentxy is None or self.currentxy == (x, y):
+            self.setcurrent(x, y)
+            return
+        self.clearfocus()
+        self.cornerxy = x, y
+        x1, y1 = self.currentxy
+        x2, y2 = self.cornerxy or self.currentxy
+        if x1 > x2:
+            x1, x2 = x2, x1
+        if y1 > y2:
+            y1, y2 = y2, y1
+        for x in range(x1, x2+1):
+            for y in range(y1, y2+1):
+                gridcell = self.gridcells.get((x, y))
+                if gridcell is not None:
+                    gridcell['bg'] = 'lightBlue'
+        name1 = cellname(*self.currentxy)
+        name2 = cellname(*self.cornerxy)
+        self.beacon['text'] = "%s:%s" % (name1, name2)
+
+
+    def clearfocus(self):
+        if self.currentxy is not None:
+            x1, y1 = self.currentxy
+            x2, y2 = self.cornerxy or self.currentxy
+            if x1 > x2:
+                x1, x2 = x2, x1
+            if y1 > y2:
+                y1, y2 = y2, y1
+            for x in range(x1, x2+1):
+                for y in range(y1, y2+1):
+                    gridcell = self.gridcells.get((x, y))
+                    if gridcell is not None:
+                        gridcell['bg'] = 'white'
+
+    def return_event(self, event):
+        "Callback for the Return key."
+        self.change_cell()
+        x, y = self.currentxy
+        self.setcurrent(x, y+1)
+        return "break"
+
+    def shift_return_event(self, event):
+        "Callback for the Return key with Shift modifier."
+        self.change_cell()
+        x, y = self.currentxy
+        self.setcurrent(x, max(1, y-1))
+        return "break"
+
+    def tab_event(self, event):
+        "Callback for the Tab key."
+        self.change_cell()
+        x, y = self.currentxy
+        self.setcurrent(x+1, y)
+        return "break"
+
+    def shift_tab_event(self, event):
+        "Callback for the Tab key with Shift modifier."
+        self.change_cell()
+        x, y = self.currentxy
+        self.setcurrent(max(1, x-1), y)
+        return "break"
+
+    def change_cell(self):
+        "Set the current cell from the entry widget."
+        x, y = self.currentxy
+        text = self.entry.get()
+        cell = None
+        if text.startswith('='):
+            cell = FormulaCell(text[1:])
+        else:
+            for cls in int, long, float, complex:
+                try:
+                    value = cls(text)
+                except:
+                    continue
+                else:
+                    cell = NumericCell(value)
+                    break
+        if cell is None and text:
+            cell = StringCell(text)
+        if cell is None:
+            self.sheet.clearcell(x, y)
+        else:
+            self.sheet.setcell(x, y, cell)
+        self.sync()
+
+    def sync(self):
+        "Fill the GUI cells from the sheet cells."
+        self.sheet.recalc()
+        for (x, y), gridcell in self.gridcells.iteritems():
+            if x == 0 or y == 0:
+                continue
+            cell = self.sheet.getcell(x, y)
+            if cell is None:
+                gridcell['text'] = ""
+            else:
+                if hasattr(cell, 'format'):
+                    text, alignment = cell.format()
+                else:
+                    text, alignment = str(cell), LEFT
+                gridcell['text'] = text
+                gridcell['anchor'] = align2anchor[alignment]
+
+
+def test_basic():
+    "Basic non-gui self-test."
+    import os
+    a = Sheet()
+    for x in range(1, 11):
+        for y in range(1, 11):
+            if x == 1:
+                cell = NumericCell(y)
+            elif y == 1:
+                cell = NumericCell(x)
+            else:
+                c1 = cellname(x, 1)
+                c2 = cellname(1, y)
+                formula = "%s*%s" % (c1, c2)
+                cell = FormulaCell(formula)
+            a.setcell(x, y, cell)
+##    if os.path.isfile("sheet1.xml"):
+##        print "Loading from sheet1.xml"
+##        a.load("sheet1.xml")
+    a.display()
+    a.save("sheet1.xml")
+
+def test_gui():
+    "GUI test."
+    g = SheetGUI()
+    g.root.mainloop()
+
+if __name__ == '__main__':
+    #test_basic()
+    test_gui()