come_from_opt handles and/or precidence properly

main.py: give a better error message when file is not found.

test/simple_source/comprehension/10-list-ifnot.py

@@ -0,0 +1,17 @@
+# Test semantic handling of
+# [x for x in names if not y]
+import os
+
+def bug(dirname, pattern):
+    if not dirname:
+        if isinstance(pattern, bytes):
+            dirname = bytes(os.curdir, 'ASCII')
+        else:
+            dirname = os.curdir
+    try:
+        names = os.listdir(dirname)
+    except os.error:
+        return []
+    if not _ishidden(pattern):
+        names = [x for x in names if not _ishidden(x)]
+    return fnmatch.filter(names, pattern)