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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def contract_variables(self, u, v):
 """Enforce u, v being the same variable in a binary quadratic model. The resulting variable is labeled 'u'. Values of interactions between `v` and variables that `u` interacts with are added to the corresponding interactions of `u`. Args: u (variable):
 Variable in the binary quadratic model. v (variable):
 Variable in the binary quadratic model. Examples: This example creates a binary quadratic model representing the K4 complete graph and contracts node (variable) 3 into node 2. The interactions between 3 and its neighbors 1 and 4 are added to the corresponding interactions between 2 and those same neighbors. False 25 """

        adj = self.adj

        if u not in adj:
            raise ValueError("{} is not a variable in the binary quadratic model".format(u))
        if v not in adj:
            raise ValueError("{} is not a variable in the binary quadratic model".format(v))

        # if there is an interaction between u, v it becomes linear for u
        if v in adj[u]:
            if self.vartype is Vartype.BINARY:
                self.add_variable(u, adj[u][v])
            elif self.vartype is Vartype.SPIN:
                self.add_offset(adj[u][v])
            else:
                raise RuntimeError("unexpected vartype")
            self.remove_interaction(u, v)

        # all of the interactions that v has become interactions for u
        neighbors = list(adj[v])
        for w in neighbors:
            self.add_interaction(u, w, adj[v][w])
            self.remove_interaction(v, w)

        # finally remove v
        self.remove_variable(v)