Added similarity relation.

src/main.rs

@@ -5,5 +5,9 @@ mod tree;
 mod test;
 
 fn main() {
-  unimplemented!();
+  // safely: trust me!!
+  // having fun or funly having?
+  unsafe {
+    std::hint::unreachable_unchecked()
+  }
 }

src/tree.rs

@@ -93,3 +93,19 @@ impl<T: Eq, U: Float> Tree<T, U> {
   }
 }
 
+pub struct SimilarityRelation<T: Eq> {
+  left: T,
+  right: T,
+  with_respect_to: T
+}
+
+// implement symmetry
+impl<T: Eq> PartialEq for SimilarityRelation<T> {
+  fn eq(&self, other: &Self) -> bool {
+    (
+      (self.left == other.left && self.right == other.right)
+        || (self.left == other.right && self.right == other.left)
+    )
+    && self.with_respect_to == other.with_respect_to
+  }
+}