P83 - Construct all spanning trees

Write a function (s-tree graph) to construct (by backtracking) all spanning trees of a given graph. With this function, find out how many spanning trees there are for the graph depicted to the left. The data of this example graph can be found in the file p83.dat.

When you have a correct solution for the s-tree function, use it to define two other useful functions: (is-tree graph) and (is-connected graph). Both are five-minutes tasks!