For each instance of the node where the selector is defined, it
is evaluated, and for each node selected by the selector, a
tuple is constructed by evaluating the 'tailf:unique-leaf'
expression. All such tuples must be unique. If a
'tailf:unique-leaf' expression refers to a non-existing leaf,
the corresponding tuple is ignored.
For each instance of the node where the selector is defined, it
is evaluated, and for each node selected by the selector, a
tuple is constructed by evaluating the 'tailf:unique-leaf'
expression. All such tuples must be unique. If a
'tailf:unique-leaf' expression refers to a non-existing leaf,
the corresponding tuple is ignored.