diff --git a/src/main.cpp b/src/main.cpp
index dd11c62..ffefaba 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -228,6 +228,16 @@ class tree
 class gp_population
 {
     public:
+        enum class crossover_error_t
+        {
+            
+        };
+        struct selection
+        {
+            blt::size_t index;
+            tree* t_ref;
+        };
+        
         struct gp_i
         {
             std::unique_ptr<tree> t = nullptr;
@@ -262,9 +272,10 @@ class gp_population
             evaluate_population();
         }
         
-        [[nodiscard]] static tree* tournament_select(population_storage_t& p)
+        [[nodiscard]] static selection tournament_select(population_storage_t& p)
         {
             tree* n = nullptr;
+            blt::size_t ni = 0;
             float fitness = -2 * 8192;
             for (int i = 0; i < TOURNAMENT_SIZE; i++)
             {
@@ -272,18 +283,18 @@ class gp_population
                 do
                 {
                     index = random_individual_index();
-                    auto& v = p[index];
-                    if (v.fitness >= fitness)
-                    {
-                        n = v.t.get();
-                        fitness = v.fitness;
-                    }
-                    //BLT_TRACE("%d: %p -> %f ? %p -> %f || %d %d", index, v.t.get(), v.fitness, n, fitness, n != v.t.get(), v.fitness > fitness);
                 } while (n == p[index].t.get());
+                
+                auto& v = p[index];
+                if (v.fitness >= fitness)
+                {
+                    n = v.t.get();
+                    ni = index;
+                    fitness = v.fitness;
+                }
             }
-            //BLT_DEBUG("%p -> %f", n, fitness);
             BLT_ASSERT(n != nullptr);
-            return n;
+            return {ni, n};
         }
         
         void run_step()
@@ -302,49 +313,41 @@ class gp_population
             blt::size_t crossover_count = 0;
             blt::size_t mutation_count = 0;
             BLT_TRACE("Running Crossover");
-            for (blt::size_t i = insert_pos; i < POPULATION_SIZE; i++)
+            for (blt::size_t i = 0; i < POPULATION_SIZE; i++)
             {
-                if (insert_pos >= POPULATION_SIZE)
-                    break;
                 if (rand(engine) < CROSSOVER_RATE)
                 {
-                    tree* p1 = select();
-                    tree* p2;
+                    selection p1 = tournament_select(new_pop);
+                    selection p2{};
                     do
                     {
-                        p2 = select();
-                    } while (p2 == p1);
+                        p2 = tournament_select(new_pop);
+                    } while (p2.t_ref == p1.t_ref);
                     
-                    if (auto r = tree::crossover(p1, p2))
+                    if (auto r = tree::crossover(p1.t_ref, p2.t_ref))
                     {
-                        new_pop[insert_pos++] = {std::move(r->c1)};
-                        new_pop[insert_pos++] = {std::move(r->c2)};
+                        new_pop[p1.index] = {std::move(r->c1)};
+                        new_pop[p2.index] = {std::move(r->c2)};
                         crossover_count++;
                     }
                 }
             }
             
             BLT_TRACE("Running Mutation");
-            for (blt::size_t i = insert_pos; i < POPULATION_SIZE; i++)
+            for (blt::size_t i = 0; i < POPULATION_SIZE; i++)
             {
-                if (insert_pos >= POPULATION_SIZE)
-                    break;
                 if (rand(engine) < MUTATION_RATE)
                 {
-                    auto* p1 = select();
+                    auto p1 = tournament_select(new_pop);
                     
-                    if (auto r = tree::mutate(p1))
+                    if (auto r = tree::mutate(p1.t_ref))
                     {
-                        new_pop[insert_pos++] = gp_i{std::move(r->c), 0};
+                        new_pop[p1.index] = gp_i{std::move(r->c), 0};
                         mutation_count++;
                     }
                 }
             }
             
-            while (insert_pos < POPULATION_SIZE)
-            {
-                new_pop[insert_pos++] = {select()->clone()};
-            }
             pop = std::move(new_pop);
             BLT_TRACE("ran %d crossovers and %d mutations", crossover_count, mutation_count);
         }
@@ -414,6 +417,25 @@ void print_bits(float f)
     std::cout << std::endl;
 }
 
+enum class error
+{
+    S1, S2, S3
+};
+
+struct simple
+{
+    float f;
+};
+
+blt::expected<float, error> t1(bool b)
+{
+    float f = 50.0f * b;
+    if (b)
+        return f;
+    else
+        return error::S1;
+}
+
 void init()
 {
     global_matrices.create_internals();