-> Result<Option<Image>, ()> {
    if input.try_parse(|input| input.expect_ident_matching("none")).is_ok() {
        Ok(None)
    } else {
        Image::parse(context, input).map(Some)  // tail call
    }
}
```

```rust,ignore
// [ <length> | <percentage> ] [ <length> | <percentage> ]?
fn parse_border_spacing(_context: &ParserContext, input: &mut Parser)
                          -> Result<(LengthOrPercentage, LengthOrPercentage), ()> {
    let first = LengthOrPercentage::parse?;
    let second = input.try_parse(LengthOrPercentage::parse).unwrap_or(first);
    (first, second)
}
```