Make id the document identifier to be used in the next
xpath evaluation without identifier prefix.
Example 1.
xsh> a=mydoc1.xml# opens and selects a
xsh> ls /# lists a
xsh> b=mydoc2.xml# opens and selects b
xsh> ls /# lists b
xsh> ls a:/# lists and selects a
xsh> select b# does nothing except selecting b
xsh> ls /# lists b