addZero now works

distinguish two modes of unification
while pattern matching we return constraints on variables, and normally
we are more aggressive about evaluating when matching against a variable.
fixes to `let`
surface #check in vscode

src/Lib/Tokenizer.idr

@@ -23,7 +23,7 @@ identMore : Lexer
 identMore = alphaNum <|> exact "." <|> exact "'" <|> exact "_"
 
 singleton : Lexer
-singleton = oneOf "()\\{}[],"
+singleton = oneOf "()\\{}[],?"
 
 quo : Recognise True
 quo = is '"'