21.   Pakistan closed its porous border to all but those Afghans fortunate enough to hold official visas.

22.   Porous borders and open competition have also proven to be a double-edged sword.

23.   The new terrorist cells became a huge intelligence headache, since many carried out one attack, dissolved, and reformed with new members across the porous European borders.

24.   The reason, of course, is the porous border with the United States.

25.   Their porous common border has been a fluid and often-ignored barrier until now.

26.   They also appear impotent to stem the flows of migrants across their increasingly porous borders.

27.   They are clouded by old suspicions, problems of illegal immigration across the now porous border, and sometimes plain prejudice.

28.   This is to say nothing of the porous border with Afghanistan that remains largely unpatrolled.

29.   U.S. plans to beef up vigilance at the porous Dominican border may rob her again of her ability to support her three children.