diff --git a/aiger/common.py b/aiger/common.py
index <HASH>..<HASH> 100644
--- a/aiger/common.py
+++ b/aiger/common.py
@@ -112,7 +112,10 @@ def sink(inputs):
         comments=('sink', ))
 
 
-def tee(outputs):
+def tee(outputs=None):
+    if not outputs:
+        return empty()
+
     def tee_output(name, renames):
         return frozenset((r, aig.Input(name)) for r in renames)
 
diff --git a/aiger/parser.py b/aiger/parser.py
index <HASH>..<HASH> 100755
--- a/aiger/parser.py
+++ b/aiger/parser.py
@@ -29,7 +29,7 @@ symbol_kind = ("i" / "o" / "l")
 symbol_name = (~r".")+
 
 comments = "c" EOL comment+
-comment = (~r".")+ EOL?
+comment = (~r".")* EOL?
 
 _ = ~r" "+
 id = ~r"\d"+
diff --git a/setup.py b/setup.py
index <HASH>..<HASH> 100644
--- a/setup.py
+++ b/setup.py
@@ -25,7 +25,6 @@ setup(
         'parsimonious',
         'dd',
         'toposort',
-        'lens',
     ],
     packages=find_packages(),
 )
extend tee to support empty mapping + remove lens dependency