Twig.php follows the same rules for printing booleans as PHP does.

In PHP "false" is printed as empty string. "true" is printed as 1

{{ 1 and 0 }}

Print:

0

should print:

I'm not sure how to fix this.
I should change Twig.expression.operator.parse to add the PHP-like strings, but isn't that function both used for outputting text and the evaluating operator expressions?

I could also, make Twig.expression.operator.parse add the proper types to the stack and then parse the output at the end (the expression "a && b" add a 1 if true and 0 if false), but I don't understand the code well enough to see if this is a good idea or not.