diff --git a/mission_rust/src/fsrc/introspection.rs b/mission_rust/src/fsrc/introspection.rs
new file mode 100644
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+++ b/mission_rust/src/fsrc/introspection.rs
@@ -0,0 +1,63 @@
+/// A trait to allow objects to be queried for some of their inner attributes at runtime.
+/// 
+pub trait Introspection {
+    /// Executes a function/closure on all members of the implementing struct.
+    /// 
+    /// The parameters for the closure a reference to the member as well as the name of the member.
+    /// 
+    /// To keep the trait object safe, we need to pass the closure as a reference. Otherwise,
+    /// the trait can not be used as a `dyn` reference, because the function becomes generic.
+    /// (Either using `impl FnMut` not `where F: FnMut` will be treated as generic making the
+    /// trait not object safe).
+    /// Additionally, the closure is typed as `FnMut`, to allow the closure to capture mutably.
+    /// 
+    /// 
+    /// # Examples
+    /// 
+    /// ```rust
+    /// use fsrc::introspection::Introspection;
+    /// 
+    /// struct Empty {}
+    /// 
+    /// impl Empty {
+    ///     fn getRandomNumber(&self) -> u8 {
+    ///         return 4    // chosen by fair dice roll.
+    ///                     // guaranteed to be random.
+    ///     }
+    /// }
+    /// 
+    /// fn parsing_something(something: &dyn Introspection) {
+    ///     let mut a = 0;
+    ///     something.for_each_member(&mut |x, _| {
+    ///         if let Some( empty) = x.downcast_ref::<Empty>() {
+    ///             a = empty.getRandomNumber();
+    ///         }
+    ///     });
+    /// }
+    /// ````
+    ///
+    fn for_each_member(&self, f: &mut dyn FnMut(&dyn core::any::Any, &str) -> ());
+
+    /// Same as [for_each_member](Introspection::for_each_member), only with a return value for the closure.
+    /// 
+    /// # Examples
+    /// 
+    /// The return value allows using `x.downcast_ref()?` or similar constructs in the closure like in:
+    /// 
+    /// ```rust
+    /// use fsrc::introspection::Introspection;
+    /// 
+    /// struct Empty {}
+    /// 
+    /// fn do_something(_: &Empty) {}
+    /// 
+    /// fn parsing_something(something: &dyn Introspection) {
+    ///     something.for_each_member_return(&mut |x, _| {
+    ///         do_something(x.downcast_ref()?);
+    ///         None
+    ///     });
+    /// }
+    /// ```
+    /// 
+    fn for_each_member_return(&self, f: &mut dyn FnMut(&dyn core::any::Any, &str) -> Option<()>);
+}
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