diff --git a/bin/tools/expression.py b/bin/tools/expression.py
index e48668408ba..c219976ac12 100644
--- a/bin/tools/expression.py
+++ b/bin/tools/expression.py
@@ -1,36 +1,61 @@
 #!/usr/bin/env python
-
-def _is_operator( element ):
-    return isinstance( element, str ) and element in ['&','|']
-
-def _is_leaf( element ):
-    return isinstance( element, tuple ) and len( element ) == 3 and element[1] in ['=', '<>', '!=', '<=', '<', '>', '>=', 'like', 'not like', 'ilike', 'not ilike'] 
-
-def _is_expression( element ):
-    return isinstance( element, tuple ) and len( element ) > 2 and _is_operator( element[0] )
-
-class expression_leaf( object ):
-    def __init__(self, operator, left, right ):
-        self.operator = operator
-        self.left = left
-        self.right = right
-
-    def parse( self ):
-        return self
-
-    def to_sql( self ):
-        return "%s %s %s" % ( self.left, self.operator, self.right )
+# -*- encoding: utf-8 -*-
 
 class expression( object ):
+    """
+    parse a domain expression
+    examples:
+
+    >>> e = [('foo', '=', 'bar')]
+    >>> expression(e).parse().to_sql()
+    'foo = bar'
+    >>> e = [('field', '=', 'value'), ('field', '!=', 'value')]
+    >>> expression(e).parse().to_sql()
+    '( field = value AND field != value )'
+    >>> e = [('&', ('field', '<', 'value'), ('field', '>', 'value'))]
+    >>> expression(e).parse().to_sql()
+    '( field < value AND field > value )'
+    >>> e = [('|', ('field', '=', 'value'), ('field', '=', 'value'))]
+    >>> expression(e).parse().to_sql()
+    '( field = value OR field = value )'
+    >>> e = [('&', ('field1', '=', 'value'), ('field2', '=', 'value'), ('|', ('field3', '<>', 'value'), ('field4', '=', 'value')))]
+    >>> expression(e).parse().to_sql()
+    '( field1 = value AND field2 = value AND ( field3 <> value OR field4 = value ) )'
+    >>> e = [('&', ('|', ('a', '=', '1'), ('b', '=', '2')), ('|', ('c', '=', '3'), ('d', '=', '4')))]
+    >>> expression(e).parse().to_sql()
+    '( ( a = 1 OR b = 2 ) AND ( c = 3 OR d = 4 ) )'
+    >>> e = [('|', (('a', '=', '1'), ('b', '=', '2')), (('c', '=', '3'), ('d', '=', '4')))]
+    >>> expression(e).parse().to_sql()
+    '( ( a = 1 AND b = 2 ) OR ( c = 3 AND d = 4 ) )'
+    >>> expression('fail').parse().to_sql()
+    Traceback (most recent call last):
+    ...
+    ValueError: Bad expression: 'fail'
+    >>> e = [('fail', 'is', 'True')]
+    >>> expression(e).parse().to_sql()
+    Traceback (most recent call last):
+    ...
+    ValueError: Bad expression: ('&', ('fail', 'is', 'True'))
+    """
+
+    def _is_operator( self, element ):
+        return isinstance( element, str ) and element in ['&','|']
+
+    def _is_leaf( self, element ):
+        return isinstance( element, tuple ) and len( element ) == 3 and element[1] in ['=', '<>', '!=', '<=', '<', '>', '>=', 'like', 'not like', 'ilike', 'not ilike'] 
+
+    def _is_expression( self, element ):
+        return isinstance( element, tuple ) and len( element ) > 2 and self._is_operator( element[0] )
+
     def __init__( self, exp ):
         if isinstance( exp, tuple ):
-            if not _is_leaf( exp ) and not _is_operator( exp[0] ):
+            if not self._is_leaf( exp ) and not self._is_operator( exp[0] ):
                 exp = list( exp )
         if isinstance( exp, list ):
-            if len( exp ) == 1 and _is_leaf( exp[0] ):
+            if len( exp ) == 1 and self._is_leaf( exp[0] ):
                 exp = exp[0]
             else:
-                if not _is_operator( exp[0][0] ):
+                if not self._is_operator( exp[0][0] ):
                     exp.insert( 0, '&' )
                     exp = tuple( exp )
                 else:
@@ -40,19 +65,31 @@ class expression( object ):
         self.operator = '&'
         self.children = []
 
+        self.left, self.right = None, None
+        if self._is_leaf(self.exp):
+            self.left, self.operator, self.right = self.exp
+        elif not self._is_expression( self.exp ):
+            raise ValueError, 'Bad expression: %r' % (self.exp,)
+
     def parse( self ):
-        if _is_leaf( self.exp ):
-            self.children.append( expression_leaf( self.exp[1], self.exp[0], self.exp[2] ).parse() )
-        elif _is_expression( self.exp ):
+       if not self._is_leaf( self.exp ) and self._is_expression( self.exp ):
             self.operator = self.exp[0]
 
             for element in self.exp[1:]:
-                if not _is_operator( element ) and not _is_leaf(element):
+                if not self._is_operator( element ):
                     self.children.append( expression(element).parse() )
-                else:
-                    if _is_leaf(element):
-                        self.children.append( expression_leaf( element[1], element[0], element[2] ).parse() )
-        return self
+       return self
 
     def to_sql( self ):
-        return "( %s )" % ((" %s " % {'&' : 'AND', '|' : 'OR' }[self.operator]).join([child.to_sql() for child in self.children]))
+        if self._is_leaf( self.exp ):
+            return "%s %s %s" % ( self.left, self.operator, self.right )
+        else:
+            return "( %s )" % (" %s " % {'&' : 'AND', '|' : 'OR' }[self.operator]).join([child.to_sql() for child in self.children])
+
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()
+
+# vim:expandtab:smartindent:tabstop=4:softtabstop=4:shiftwidth=4:
+