Will a ceasefire in Syria hold?

After years of fighting and months of diplomatic wrangling, the US and Russia have agreed to a so-called "cessation of hostilities" in Syria. 

It is supposed to start first thing on Saturday and requires both countries to persuade their allies on the ground to stop fighting. 

But there are major problems. The agreement excludes the Islamic State of Iraq and the Levant (ISIL) and al-Nusra Front and allows air strikes to continue against both of those groups. 

And Syrian rebels say the exclusion of al-Nusra gives Russia a pretext to continue to attack areas where various rebel groups are tightly packed.