It's for LaTeX formatting in the first result card; by default it checks if the top-level AST node is a plain function (not derived from SymPy's `Function`), and if so, formats it as `\mathrm{functionname}(arguments)` in LaTeX, just to make it look nicer. But `sqrt` gets caught in this as well, when I'd rather have it formatted as `\sqrt{}`, so this is just a blacklist.