If one makes his succah on top of a wagon or on the deck of a ship it is valid, and we may go up into it on the festival. [If he makes a succah] in the top of a tree or on the back of a camel it is valid, but we may not go up into it on the festival.  If two [walls are] in a tree and one is man-made, or two [are] man-made and one [is] in a tree, it is valid, but we may not go up into it on the festival. If three [walls are] man-made and one [is] in a tree, it is valid, and we may go up into it on the festival.  This is the general rule: Wherever the tree can be removed and [the succah] can stand by itself, it is valid, and we may go up into it on the festival.