XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (314 page)

Note that the effective boolean value of a sequence doesn't simply test whether the sequence is empty, because of the special cases for a singleton sequence. If you want to test whether a sequence is empty, use the
empty()
or
exists()
functions described in Chapter 13.

These rules for forming the effective boolean value are consistent with the rules used in other XPath contexts where a true/false value is required. These include:

• The operands of
  and
  and
  or
  (see Chapter 8)
  
• The argument of the functions
  boolean()
  and
  not()
  (see Chapter 13)
  
• The expression used as a predicate within square brackets in an axis step or filter expression, so long as the value is not numeric (see Chapters 9 and 10)
  
• The expression in the
  satisfies
  clause of
  some
  and
  every
  expressions (see Chapter 10)
  

The same rules are also used in XSLT 2.0, in evaluating the

and

instructions.

However, the rules are not the same as the rules in XML Schema for converting a string to an
xs:boolean
value. In XML Schema, the valid lexical representations of the
xs:boolean
value
false
are
0
and
false
, while the valid lexical representations of
true
are
1
and
true
. The XML Schema rules are used in XPath 2.0 in the following circumstances: