Sequence size Functions

fn:zero-or-one($arg as item()*) => item()?

Returns $arg if it contains zero or one items. Otherwise, raises an error

fn:one-or-more($arg as item()*) => item()?

Returns $arg if it containsone or more items. Otherwise, raises an error

fn:exactly-one($arg as item()*) => item()?

Returns $arg if it contains exactly one item. Otherwise, raises an error