diff -r 4a40718c7b4b -r b538e1f7011c lib/tree_parse.py
--- a/lib/tree_parse.py	Sat Feb 07 02:46:58 2009 +0200
+++ b/lib/tree_parse.py	Sat Feb 07 02:51:36 2009 +0200
@@ -5,6 +5,13 @@
     A file consists of a number of lines, and each line consists of indenting whitespace and data. Each line has a parent
 """
 
+class TreeParseError (Exception) :
+    """
+        Error parsing a tree file
+    """
+
+    pass
+
 def _read_lines (path, stop_tokens='') :
     """
         Reads lines from the given path, ignoring empty lines, and yielding (line_number, indent, line) tuples, where 
@@ -109,7 +116,7 @@
                         break
 
                     elif stack_indent < indent :
-                        assert False, "Bad un-indent"
+                        raise TreeParseError("Bad unindent on %s:%d, %d < %d" % (path, line_number, stack_indent, indent))
         
         # add to parent?
         if parent :