fmap unparse $ expr (lexer "a+b+c+d") :: Maybe String
fmap unparse $ expr (lexer "a+b*c+d") :: Maybe String
fmap unparse $ expr (lexer "a+b*c*d") :: Maybe String
fmap unparse $ expr (lexer "a+b+-c+d") :: Maybe String
fmap unparse $ expr (lexer "a+-b*c+d") :: Maybe String