<< Handling non-nesting >>

• Easy: a nesting counter that is non-zero when inside a declaration.
• Pre-invalidate fragments if nesting>0 on fragment enter or exit.
• Better: combine fragments. If nesting>0 on fragment boundary, combine neighboring fragments into one.
• Difficulty: must test conditionals before first combined fragment.
• Dis-allowed if conditional moved across #define, #undef or #include.