Thematic breaks can't have an indent of more than 3

src/Components/Blocks/Rule.php

@@ -24,6 +24,10 @@ final class Rule implements Block
         Block $Block = null,
         State $State = null
     ) {
+        if ($Context->line()->indent() > 3) {
+            return null;
+        }
+
         $marker = $Context->line()->text()[0];
 
         if (\substr_count($Context->line()->text(), $marker) >= 3 and \chop($Context->line()->text(), " $marker") === '') {

tests/data/horizontal_rule.html

@@ -3,3 +3,5 @@
 <hr />
 <hr />
 <hr />
+<p>foo
+***</p>

tests/data/horizontal_rule.md

@@ -7,3 +7,6 @@
 ***
 
 ___
+
+foo
+    ***