[beagle3]

Tabs are fine and spaces are fine, but mixing tabs and spaces for indentation is a recipe for disaster, because different environments and tools have a different tab width setting -- as a result, "printing a file and OCRing it" (which people do, e.g. when typing code shown somewhere else) can result in legal but semantically very different programs looking the same. That's why you shouldn't use tabs on code that is ever edited/viewed by more than one person[0], and why you should always use python with -tt mode.

Nim takes a practical approach by banning tabs altogether; The "#?" hack you suggest just shows another great Nim feature - source code filters are standardized; You don't need a preprocessor/lex/yacc/re2c/swig with its own driver/makefile; it's all well documented and tracked within your Nim environment.

[0] especially in languages in which indentation changes program semantics, but even in e.g. C - where mismatch between indentation and curly brackets can let a bug like "goto fail;" hide in plain sight.