Nothing, we can compile x*3. We can't compile x**3, because we do not have a runtime library setup for WASM yet (Flang above had the same issue) and WASM can do x**2, but arbitrary power, such as x**3, requires a runtime power function that we haven't implemented yet. If you want to help, you can fix it probably quite easily right here: https://github.com/lfortran/lfortran/blob/69d488b1d1fd26b163....
