Boolean expressions: AND and OR
E1 and E2 returns true if the effective boolean values
E2 are both true.
E1 or E2 returns true if the effective boolean values of
either or both of
E2 are true.
The specification allows the operands to be evaluated in either order, but Saxon will
normally evaluate them left-to-right, which means if you write something like
instance of xs:integer and $x = (1 to $a), then you won't get an error if
$a is not an integer. But the specification doesn't guarantee this.