diff --git a/include/core.hpp b/include/core.hpp
index 61c6749..0d82104 100644
--- a/include/core.hpp
+++ b/include/core.hpp
@@ -14,6 +14,6 @@ namespace riddle
     core() noexcept;
     virtual ~core() = default;
 
-    [[nodiscard]] std::optional<std::reference_wrapper<item>> get(std::string_view name) noexcept override;
+    [[nodiscard]] std::shared_ptr<item> get(std::string_view name) noexcept override;
   };
 } // namespace riddle
diff --git a/include/env.hpp b/include/env.hpp
index a9a7d1d..6c978b8 100644
--- a/include/env.hpp
+++ b/include/env.hpp
@@ -4,7 +4,6 @@
 #include <map>
 #include <string>
 #include <string_view>
-#include <optional>
 
 namespace riddle
 {
@@ -26,28 +25,28 @@ namespace riddle
   class env
   {
   public:
-    env(core &c, env &parent, std::map<std::string, std::unique_ptr<item>, std::less<>> &&items = {}) noexcept;
+    env(core &c, env &parent, std::map<std::string, std::shared_ptr<item>, std::less<>> &&items = {}) noexcept;
     virtual ~env() = default;
 
     /**
      * @brief Retrieves an item by its name.
      *
-     * This function searches for an item with the specified name and returns it
-     * wrapped in a std::optional. If the item is found, it is returned as a
-     * std::reference_wrapper. If the item is not found, an empty std::optional
-     * is returned.
+     * This function retrieves an item by its name. The function will first
+     * search for the item in the environment's items. If the item is not found,
+     * the function will search for the item in the parent environment. This
+     * process continues until the item is found or there are no more parent
+     * environments to search.
      *
      * @param name The name of the item to retrieve.
-     * @return std::optional<std::reference_wrapper<item>> The item wrapped in
-     * a std::optional if found, otherwise an empty std::optional.
+     * @return The item with the given name, or nullptr if the item is not found.
      */
-    [[nodiscard]] virtual std::optional<std::reference_wrapper<item>> get(std::string_view name) noexcept;
+    [[nodiscard]] virtual std::shared_ptr<item> get(std::string_view name) noexcept;
 
   private:
     core &cr;
     env &parent;
 
   protected:
-    std::map<std::string, std::unique_ptr<item>, std::less<>> items;
+    std::map<std::string, std::shared_ptr<item>, std::less<>> items;
   };
 } // namespace riddle
diff --git a/include/expression.hpp b/include/expression.hpp
new file mode 100644
index 0000000..f27a879
--- /dev/null
+++ b/include/expression.hpp
@@ -0,0 +1,15 @@
+#pragma once
+
+#include "item.hpp"
+
+namespace riddle
+{
+  class expression
+  {
+  public:
+    expression() = default;
+    virtual ~expression() = default;
+
+    [[nodiscard]] virtual std::shared_ptr<item> evaluate(env &e) const = 0;
+  };
+} // namespace riddle
diff --git a/src/core.cpp b/src/core.cpp
index 19c35d3..bc43a64 100644
--- a/src/core.cpp
+++ b/src/core.cpp
@@ -5,10 +5,10 @@ namespace riddle
 {
     core::core() noexcept : env(*this, *this) {}
 
-    std::optional<std::reference_wrapper<item>> core::get(std::string_view name) noexcept
+    std::shared_ptr<item> core::get(std::string_view name) noexcept
     {
         if (auto it = items.find(name); it != items.end())
-            return *it->second;
-        return std::nullopt;
+            return it->second;
+        return nullptr;
     }
 } // namespace riddle
\ No newline at end of file
diff --git a/src/env.cpp b/src/env.cpp
index 99ac93f..a6c630d 100644
--- a/src/env.cpp
+++ b/src/env.cpp
@@ -4,12 +4,12 @@
 
 namespace riddle
 {
-    env::env(core &c, env &parent, std::map<std::string, std::unique_ptr<item>, std::less<>> &&items) noexcept : cr(c), parent(parent), items(std::move(items)) {}
+    env::env(core &c, env &parent, std::map<std::string, std::shared_ptr<item>, std::less<>> &&items) noexcept : cr(c), parent(parent), items(std::move(items)) {}
 
-    std::optional<std::reference_wrapper<item>> env::get(std::string_view name) noexcept
+    std::shared_ptr<item> env::get(std::string_view name) noexcept
     {
         if (auto it = items.find(name); it != items.end())
-            return *it->second;
+            return it->second;
         else
             return parent.get(name);
     }