Function interp_is_virtual_class 

fn interp_is_virtual_class(
    args: &[RValue],
    _named: &[(String, RValue)],
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Check if a class is virtual.

Looks up the class in the S4 registry and returns its virtual status.

@param Class character string naming the class @return TRUE if the class is virtual, FALSE otherwise