The following names cannot be used for top-level variables without square-bracket syntax (as .vars["in"]), since they are keywords in FTL:
- true: boolean value ``true''
- false: boolean value ``false''
- gt: comparison operator ``greater than''
- gte: comparison operator ``greater than or equivalent''
- lt: comparison operator ``less than''
- lte: comparison operator ``less than or equivalent''
- as: used by a few directives
- in: used by a few directives
- using: used by a few directives