diff --git a/builder.py b/builder.py
index 3b33912..5c6aa34 100644
--- a/builder.py
+++ b/builder.py
@@ -13,7 +13,7 @@ def _default_do_files(filename):
         yield ("default%s.do" % ext), basename, ext
     
 
-def _possible_do_files(t):
+def possible_do_files(t):
     dirname,filename = os.path.split(t)
     yield (os.path.join(vars.BASE, dirname), "%s.do" % filename,
            '', filename, '')
@@ -35,8 +35,8 @@ def _possible_do_files(t):
                    subdir, os.path.join(subdir, basename), ext)
         
 
-def _find_do_file(f):
-    for dodir,dofile,basedir,basename,ext in _possible_do_files(f.name):
+def find_do_file(f):
+    for dodir,dofile,basedir,basename,ext in possible_do_files(f.name):
         dopath = os.path.join(dodir, dofile)
         debug2('%s: %s:%s ?\n' % (f.name, dodir, dofile))
         if os.path.exists(dopath):
@@ -126,7 +126,7 @@ class BuildJob:
             sf.save()
             return self._after2(0)
         sf.zap_deps1()
-        (dodir, dofile, basedir, basename, ext) = _find_do_file(sf)
+        (dodir, dofile, basedir, basename, ext) = find_do_file(sf)
         if not dofile:
             if os.path.exists(t):
                 sf.set_static()
diff --git a/redo-whichdo b/redo-whichdo
new file mode 120000
index 0000000..0962250
--- /dev/null
+++ b/redo-whichdo
@@ -0,0 +1 @@
+redo-whichdo.py
\ No newline at end of file
diff --git a/redo-whichdo.py b/redo-whichdo.py
new file mode 100755
index 0000000..03310b9
--- /dev/null
+++ b/redo-whichdo.py
@@ -0,0 +1,29 @@
+#!/usr/bin/env python
+import sys, os
+
+import vars_init
+vars_init.init([])
+
+import builder
+from log import err
+
+if len(sys.argv[1:]) != 1:
+    err('%s: exactly one argument expected.\n' % sys.argv[0])
+    sys.exit(1)
+
+want = sys.argv[1]
+for dodir,dofile,basedir,basename,ext in builder.possible_do_files(os.path.abspath(want)):
+    dopath = os.path.join('/', dodir, dofile)
+    relpath = os.path.relpath(dopath, '.')
+    exists = os.path.exists(dopath)
+    assert('\n' not in relpath)
+    if exists:
+      print '+', relpath
+      assert('\n' not in basename)
+      assert('\n' not in ext)
+      print '1', basename+ext
+      print '2', basename
+      sys.exit(0)
+    else:
+      print '-', relpath
+sys.exit(1)   # no appropriate dofile found