feat: reducer, but doesn`t work

pkg/lambda/get_free_variables.go

@@ -0,0 +1,20 @@
+package lambda
+
+import "git.maximhutz.com/max/lambda/pkg/set"
+
+func GetFreeVariables(e Expression) set.Set[string] {
+	switch e := e.(type) {
+	case *Variable:
+		return set.New(e.Value)
+	case *Abstraction:
+		vars := GetFreeVariables(e.Body)
+		vars.Remove(e.Parameter)
+		return vars
+	case *Application:
+		vars := GetFreeVariables(e.Abstraction)
+		vars.Union(GetFreeVariables(e.Argument))
+		return vars
+	default:
+		return nil
+	}
+}