restructuring the project

src/utils/isParentOf.js

@@ -0,0 +1,26 @@
+/**
+ * @module utils
+ */
+
+import { Y } from '../utils/Y.js' // eslint-disable-line
+import { Type } from '../structs/Type.js' // eslint-disable-line
+
+/**
+ * Check if `parent` is a parent of `child`.
+ *
+ * @param {Type | Y} parent
+ * @param {Type | Y} child
+ * @return {Boolean} Whether `parent` is a parent of `child`.
+ *
+ * @public
+ */
+export const isParentOf = (parent, child) => {
+  child = child._parent
+  while (child !== null) {
+    if (child === parent) {
+      return true
+    }
+    child = child._parent
+  }
+  return false
+}