selects from among all nodes in the subtree headed at the current location [?] // [id] 
descendant
c h i l d
d e s c e n d a n t
a n c e s t o r
p s i b l i n g
f s i b l i n g
p r e c e d i n g
f o l l o w i n g
a t t r
s t r i n g
s p a n