Function xml_pi 

pub fn xml_pi(
    target: &str,
    arg: Option<&str>,
    null: bool,
) -> Result<Option<String>, TypeError>

xmlpi: the processing instruction, or None when the argument is null. The check of the target comes before the check for null, as the standard says.

Errors

2200T for the target xml in any case and for an argument with ?>.