Function interp_dev_cur 

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

Return the current graphics device number.

Returns 2 if a plot is being accumulated (an “active” device), or 1 (the null device) otherwise.

@return integer device number