Predicates result in an expression value that is converted to a boolean.

If the value is a number, then the position is checked against that number.

These are equivalent:

slide[2]
slide[position()=2]

Otherwise, the boolean() function is applied to convert its value to a boolean:

• A node set is true if it is non-empty.

• A string is true if it has a non-zero length.

Boolean expressions (and, or, not ) are already boolean values.