actually replace "$$" with "$"

src/utils/parse.py

@@ -136,11 +136,15 @@ def format_tokens(s: str, names: typing.List[str], sigil: str="$"
                         tokens.append((i-1, "%s%s" % (sigil, name)))
                         i += len(name)
                         break
+            else:
+                tokens.append((i, "$"))
         i += 1
     return tokens
 
 def format_token_replace(s: str, vars: typing.Dict[str, str],
         sigil: str="$") -> str:
+    vars = vars.copy()
+    vars.update({"": ""})
     tokens = format_tokens(s, list(vars.keys()), sigil)
     tokens.sort(key=lambda x: x[0])
     tokens.reverse()