change signature of path walk to allow differently typed generators.

example/path.cpp

@@ -7,7 +7,7 @@ int main() {
     auto cube = tc::group::B(3);
     auto vars = cube.solve();
 
-    auto words = vars.path.walk<std::string>(
+    auto words = vars.path.walk<std::string, std::string>(
         "",
         {"a", "b", "c"},
         [](auto a, auto b) { return a + b; }
@@ -17,10 +17,5 @@ int main() {
         std::cout << word << std::endl;
     }
 
-//    for (size_t target = 1; target < vars.size(); target++) {
-//        auto &action = vars.path.get(target);
-//        std::cout << action.from_idx << " * " << action.gen << " = " << target << std::endl;
-//    }
-
     return 0;
 }