--- a/rdfdb/file_vs_uri.py	Sun May 15 15:49:28 2022 -0700
+++ b/rdfdb/file_vs_uri.py	Sun May 15 15:52:01 2022 -0700
@@ -1,13 +1,24 @@
 """
 note that if there are ambiguities in dirUriMap, you'll get
 undefined results that depend on python's items() order.
+
+In this example:
+  http://example.com/a/b/c <-> /my/dir/b/c.n3
+  bbbbbbbbbbbbbbbbbbbb mmm     ddddddd mmm
+The code naming goes like this:
+  b is baseUri
+  m is middle
+  d is dirpath
+
+DirUriMap is a dict of dirPath -> baseUri.
+
 """
 from pathlib import Path
-from typing import Dict
+from typing import Dict, Union
 
-from rdflib import URIRef
+from rdflib import URIRef, Namespace
 
-DirUriMap = Dict[Path, URIRef]
+DirUriMap = Dict[Path, Union[URIRef, Namespace]]
 
 
 def _pathStartsWith(fullPath: Path, possiblePrefix: Path) -> bool:
@@ -17,19 +28,20 @@
 def uriFromFile(dirUriMap: DirUriMap, filename: Path) -> URIRef:
     if not str(filename).endswith('.n3'):
         raise ValueError(filename)
-    for d, prefix in dirUriMap.items():
-        if _pathStartsWith(filename, d):
-            return URIRef(prefix + str(filename)[len(str(d)):-len(b'.n3')])
+    for dirPath, baseUri in dirUriMap.items():
+        if _pathStartsWith(filename, dirPath):
+            middle = str(filename)[len(str(dirPath)):-len('.n3')].lstrip('/')
+            return URIRef(baseUri.rstrip('/') + '/' + middle)
     raise ValueError("filename %s doesn't start with any of %s" % (filename, list(dirUriMap.keys())))
 
 
 def fileForUri(dirUriMap: DirUriMap, ctx: URIRef) -> Path:
-    for d, prefix in dirUriMap.items():
+    for dirPath, prefix in dirUriMap.items():
         if ctx.startswith(prefix):
-            middle = ctx[len(prefix):]
+            middle = ctx[len(prefix):].lstrip('/')
             if middle.endswith('/'):
                 raise NotImplementedError(f'no file mapped to {ctx=}')
-            return Path(str(d) + middle + '.n3')
+            return dirPath / (middle + '.n3')
     raise ValueError("don't know what filename to use for %s" % ctx)