test: add msubsup InvalidNumberOfChildrenError

__test__/index.test.ts

@@ -762,6 +762,16 @@ describe('#convert', () => {
 
       expect(result).toBe('\\left(x + y\\right)_{0}^{1}');
     });
+
+    describe('when it have four children', () => {
+      it('throws InvalidNumberOfChildrenError', () => {
+        const mathml = mathmlStrings.msubsupWithFourChildren;
+
+        const result = () => MathMLToLaTeX.convert(mathml);
+
+        expect(result).toThrow(new InvalidNumberOfChildrenError('msubsup', 3, 4));
+      });
+    });
   });
 
   describe('given math string with mtext', () => {

__test__/mocks/mathmlStrings.ts

@@ -1301,6 +1301,19 @@ export const msubsupWithMrow = `
 </root>
 `;
 
+export const msubsupWithFourChildren = `
+<root>
+  <math>
+    <msubsup>
+      <mo> &#x222B; </mo>
+      <mn> 0 </mn>
+      <mn> 1 </mn>
+      <mn> 5 </mn>
+    </msubsup>
+  </math>
+</root>
+`;
+
 export const mtext = `
 <root>
   <math xmlns = "http://www.w3.org/1998/Math/MathML">