diff --git a/CMakeLists.txt b/CMakeLists.txt
index bc1212e..4b03772 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -27,7 +27,7 @@ macro(compile_options target_name)
     sanitizers(${target_name})
 endmacro()
 
-project(blt-gp VERSION 0.5.14)
+project(blt-gp VERSION 0.5.15)
 
 include(CTest)
 
diff --git a/include/blt/gp/transformers.h b/include/blt/gp/transformers.h
index 36109df..94fbaf1 100644
--- a/include/blt/gp/transformers.h
+++ b/include/blt/gp/transformers.h
@@ -82,12 +82,21 @@ namespace blt::gp
             f32 terminal_chance = 0.1;
             // use traversal to select point instead of random selection
             bool traverse = false;
+
+            BLT_MAKE_SETTER_LVALUE(u32, max_crossover_tries);
+            BLT_MAKE_SETTER_LVALUE(u32, max_crossover_iterations);
+            BLT_MAKE_SETTER_LVALUE(u32, min_tree_size);
+            BLT_MAKE_SETTER_LVALUE(f32, depth_multiplier);
+            BLT_MAKE_SETTER_LVALUE(f32, terminal_chance);
+            BLT_MAKE_SETTER_LVALUE(bool, traverse);
         };
 
         explicit crossover_t(const config_t& config): config(config)
         {
         }
 
+        virtual bool apply(gp_program& program, const tree_t& p1, const tree_t& p2, tree_t& c1, tree_t& c2) = 0;
+
         [[nodiscard]] const config_t& get_config() const
         {
             return config;
diff --git a/lib/blt b/lib/blt
index a1bc8cf..2bac310 160000
--- a/lib/blt
+++ b/lib/blt
@@ -1 +1 @@
-Subproject commit a1bc8cf1c2390e507be6482dde0960daf9f662d8
+Subproject commit 2bac310e55df06a3e378c932afa2a4c2bc2123e7
diff --git a/src/program.cpp b/src/program.cpp
index 197f0e6..48b6db1 100644
--- a/src/program.cpp
+++ b/src/program.cpp
@@ -40,7 +40,7 @@ namespace blt::gp
     // this is largely to not break the tests :3
     // it's also to allow for quick setup of a gp program if you don't care how crossover or mutation is handled
     static advanced_mutation_t s_mutator;
-    static crossover_t s_crossover;
+    static subtree_crossover_t s_crossover;
     static ramped_half_initializer_t s_init;
 
     prog_config_t::prog_config_t(): mutator(s_mutator), crossover(s_crossover), pop_initializer(s_init)
diff --git a/src/transformers.cpp b/src/transformers.cpp
index 98c5a1e..d713478 100644
--- a/src/transformers.cpp
+++ b/src/transformers.cpp
@@ -139,9 +139,6 @@ namespace blt::gp
             // Mating crossover analogs to same species breeding. Only works if tree is mostly similar
         case 1:
             {
-                // if fails got to case0
-                if (false)
-                    goto case0;
             }
             // Subtree crossover, select random points inside trees and swap their subtrees
         case 2:
diff --git a/tests/serialization_test.cpp b/tests/serialization_test.cpp
index a531730..09fa1cd 100644
--- a/tests/serialization_test.cpp
+++ b/tests/serialization_test.cpp
@@ -15,6 +15,7 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
+#include <blt/std/variant.h>
 #include <filesystem>
 
 #include "../examples/symbolic_regression.h"
@@ -25,6 +26,60 @@
 #include <fstream>
 #include <blt/fs/stream_wrappers.h>
 
+
+struct no_default
+{
+    no_default() = delete;
+
+    no_default(int)
+    {
+    }
+};
+
+struct not_copyable
+{
+    not_copyable() = default;
+    not_copyable(const not_copyable&) = delete;
+};
+
+struct copyable
+{
+    copyable() = default;
+    copyable(const copyable&) = default;
+};
+
+struct copyable_nothrow
+{
+    copyable_nothrow() = default;
+    copyable_nothrow(const copyable_nothrow&) noexcept = default;
+};
+
+struct not_movable
+{
+    not_movable() = default;
+    not_movable(not_movable&&) = delete;
+};
+
+struct movable
+{
+    movable() = default;
+    movable(movable&&) = delete;
+};
+
+struct movable_nothrow
+{
+    movable_nothrow() = default;
+    movable_nothrow(movable_nothrow&&) noexcept = delete;
+};
+
+blt::variant_t<no_default> no_default_variant;
+blt::variant_t<not_copyable> not_copyable_variant;
+blt::variant_t<copyable> copyable_variant;
+blt::variant_t<copyable_nothrow> copyable_nothrow_variant;
+blt::variant_t<not_movable> not_movable_variant;
+blt::variant_t<movable> movable_variant;
+blt::variant_t<movable_nothrow> movable_nothrow_variant;
+
 using namespace blt::gp;
 
 struct context