Function subsequence 

pub fn subsequence<N: DomNavigator>(
    _context: &mut DynamicContext<'_, N>,
    args: Vec<XPathValue<N>>,
) -> Result<XPathValue<N>, XPathError>

Implements fn:subsequence - returns a contiguous subsequence.

Returns items from $sourceSeq starting at position $startingLoc and continuing for $length items (or to the end if $length is omitted).

Uses XPath 2.0 rounding rules:

• Positions are doubles, rounded to integers
• NaN startingLoc or length -> empty
• +Infinity startingLoc -> empty
• -Infinity length -> empty