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