S-expressions and dynamic binding.
They say "parentheses" but they mean the equivalence of code and data. They also say "unsafe" but they mean that they aren't used to decide themselves how and when and why evaluation happens, or to lock down the application by formal design.
Sometimes there's also a performance argument, usually it doesn't take development time into account or assumes the product will run forever without further work ever again.
Probably logic inference too, the original 'lazy generator both ways evaluating' programming style, that is used to query the database.
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