Returns a result-node-list containing
those nodes contained in rnl which are such that
proc applied to a result-node-list containing exactly that
node returns a result-node-list containing i or more
nodes. For example,
(select-by-relation (children x)
(select-elements (descendants y) "para"))))
selects those children of a node that have a descendant
element with a gi of para.
(select-by-attribute-token rnl string1 string2)
Returns a result-node-list containing those nodes in rnl
that have an attribute named string1 and that have an attribute
with a child of class attribute-value-token
with a token property equal to string2 after any
applicable string normalization.