Functions: Boolean 
boolean(object)
Description: Returns a boolean value for a number, string, or node-set. This function converts arguments to booleans according to the following rules:
If the argument is a negative or positive number, it is converted to the boolean value true. If the argument is zero or an NaN value, it is converted to false.
If the argument is a non empty node-set, it is converted to true. An empty node-set is converted to false.
If the argument is a non-empty string, it is converted to true. An empty string is converted to false.
If the argument is an object of a type other than the four basic types, it is converted to a boolean in a way that is dependent on that type.
Samples:
boolean(0) = false
boolean(1) = true
boolean(-100) = true
boolean(100) = true
boolean(NaN) = false
boolean('hello') = true
boolean('') = false
not(boolean)
Description: Returns true if the argument is false; otherwise false.
Sample:
not(true) returns false
true()
Description: Returns true
lang(string)
Description: Returns true if the language of the current node matches the language of the specified language
Sample:
lang("en") is true for <p xml:lang="en”>...</p>
lang("de") is false for <p xml:lang="en”>...</p>
false()
Description: Returns false
equal(object, object)
Description: Returns true if argument 1 equals argument 2; otherwise false
Sample:
equal(1, 2) = false
equal-or-greater(number, number)
Description: Returns true if arg1 is equal to or greater than arg2, otherwise false.
Sample:
equal-or-greater(2, 2) = true
equal-or-less(number, number)
Description: Returns true if arg1 is equal to or less than arg2; otherwise false
Sample:
equal-or-less(1, 2) = true
greater(number, number)
Description: Returns true if arg1 is greater than arg2; otherwise false
Sample:
greater(1, 2) = false
less(number, number)
Description: Returns true if arg1 is less than arg2; otherwise false
Sample:
less(1, 2) = true
logical-and(boolean, boolean)
Description: Returns true if arg1 and arg2 are both true; otherwise false
Sample:
logical-and(true, true) = true
logical-or(boolean, boolean)
Description: Returns true if arg1 or arg2 is true, otherwise false.
Sample:
logical-or(true, false) = true