Support for "not in" syntax (works in Python jinja2)

[tjsmith-meta]

The following template (foo in (...)) compiles fine in both Python jinja2 and Jinja2Cpp.

{% macro foo(val) %}
{{1 if val in ('', "''", 'foo')}}
{% endmacro %}

Whereas this slight variant (foo not in (...)) works only in Python jinja2.

{% macro foo(val) %}
{{1 if val not in ('', "''", 'foo')}}
{% endmacro %}

This seems like a bug that it doesn't work in Jinja2Cpp. I'm happy to make the code fix here, but would appreciate some pointers in the code as to what the fix might look like here. I suspect there may be a simple fix here. Help would be much appreciated!

[tjsmith-meta]

Here's a very hacky solution from a quick pass over the code. Seems unlikely to be a complete or fully correct solution, but appears like it works for the particular example template that I have.

--- a/jinja2cpp/src/expression_parser.cpp
+++ b/jinja2cpp/src/expression_parser.cpp
@@ -130,6 +130,22 @@
     default:
         switch (lexer.GetAsKeyword(tok))
         {
+        case Keyword::LogicalNot:
+        {
+            Token inToken = lexer.NextToken();
+            if (lexer.GetAsKeyword(inToken) != Keyword::In) {
+              lexer.ReturnToken();
+              return left;
+            }
+
+            auto right = ParseStringConcat(lexer);
+            if (!right) {
+              return right;
+            }
+
+            return std::make_shared<UnaryExpression>(UnaryExpression::Operation::LogicalNot,
+                std::make_shared<BinaryExpression>(BinaryExpression::In, *left, *right));
+        }
         case Keyword::In:
             operation = BinaryExpression::In;
             break;

[tjsmith-meta]

The official documentation also seems to confirm that 'not in' syntax is valid in jinja2.
https://jinja.palletsprojects.com/en/2.10.x/templates/#logic