Running wattsi in MODE="DEBUG" results in lots of assertions

A lot of:
```
Node destroyed with active exception:
EAssertionFailed exception: Assertion failed on line 753 of ropes.pas
Backtrace:
  $0000000100001591
  $0000000100011048
```
when running the HTML build script.

It took me a while to realize that it was not my newly added code that was to blame, but that this is simply the status quo.

(You can modify MODE in `src/build.sh`.)