added some explanations

package.json

@@ -29,5 +29,5 @@
   },
   "type": "module",
   "types": "dist/index.d.ts",
-  "version": "2.1.0"
+  "version": "2.1.1"
 }

readme.md

@@ -128,4 +128,36 @@ half.lt(half);  // false
 
 // <=
 half.le(half);  // true
-```
+```
+
+### destructive methods
+Methods whose names end in $ modify objects. These methods are more efficient than the methods introduced above since they do not copy objects internally.
+
+```javascript
+const half1 = new Frac(1, 2);  // (1 / 2)
+const half2 = new Frac(2, 4);  // (2 / 4)
+
+half1.add$(half2);  // (1 / 1)
+
+half1;  // (1 / 1)  <- replaced by the result of an operation
+half2;  // (2 / 4)
+```
+
+```javascript
+const frac = new Frac("(1 / (1 / 2))");  // (1 / (1 / 2))
+
+frac.reduce$();  // (2 / 1)
+
+frac;  // (2 / 1)  <- replaced by the result of an operation
+```
+
+Methods whose names end in $$ modify arguments.
+```javascript
+const half1 = new Frac(1, 2);  // (1 / 2)
+const half2 = new Frac(2, 4);  // (2 / 4)
+
+half1.add$$(half2);  // (1 / 1)
+
+half1;  // (1 / 1)  <- replaced by the result of an operation
+half2;  // (1 / 2)  <- reduced
+```

src/Frac.ts

@@ -548,6 +548,10 @@ export default class Frac {
     /**
      * Returns a string representation of the fraction.
      * 
+     * @param spaces number of whitespaces
+     * @param indent indent size of the first line
+     * @param indentLevel indent level
+     * 
      * @example
      * const half = new Frac(1, 2);  // (1 / 2)
      *