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;
 }

include/tc/core.hpp

@@ -30,11 +30,11 @@ namespace tc {
 
         void put(int from_idx, int gen, int to_idx);
 
-        template<class T>
+        template<class T, class E>
         [[nodiscard]] std::vector<T> walk(
             T start,
-            std::vector<T> gens,
-            std::function<T(const T &, const T &)> op
+            std::vector<E> gens,
+            std::function<T(const T &, const E &)> op
         ) const {
             std::vector<T> res(size());
             res[0] = start;