Add isEmpty test (deep tree check) to MmlNode and use it to test for empty mathop.

testsuite/tests/input/tex/Base.test.ts

@@ -12191,7 +12191,7 @@ describe('Character Class Changes', () => {
 
   /********************************************************************************/
 
-  it('Mathop No Apply', () => {
+  it('Mathop No Apply I', () => {
     toXmlMatch(
       tex2mml('\\mathop{} x'),
       `<math xmlns="http://www.w3.org/1998/Math/MathML" data-latex="\\mathop{} x" display="block">
@@ -12203,6 +12203,21 @@ describe('Character Class Changes', () => {
 
   /********************************************************************************/
 
+  it('Mathop no Apply II', () => {
+    toXmlMatch(
+      tex2mml('\\mathop{\\mathrm{}{}} x'),
+      `<math xmlns="http://www.w3.org/1998/Math/MathML" data-latex="\\mathop{\\mathrm{}{}} x" display="block">
+         <mrow data-mjx-texclass="OP" data-latex="\\mathop{\\mathrm{}{}}">
+           <mrow data-mjx-texclass="ORD" data-latex="\\mathrm{}"></mrow>
+           <mrow data-mjx-texclass="ORD" data-latex="{}"></mrow>
+         </mrow>
+         <mi data-latex="x">x</mi>
+       </math>`
+    );
+  });
+
+  /********************************************************************************/
+
   it('Mathrel', () => {
     toXmlMatch(
       tex2mml('\\mathrel{R}'),

ts/core/MmlTree/MmlNode.ts

@@ -144,6 +144,7 @@ export interface MmlNode extends Node<MmlNode, MmlNodeClass> {
   readonly isSpacelike: boolean;
   readonly linebreakContainer: boolean;
   readonly linebreakAlign: string;
+  readonly isEmpty: boolean;
 
   /**
    *  The expected number of children (-1 means use inferred mrow)
@@ -540,6 +541,16 @@ export abstract class AbstractMmlNode
     return 'data-align';
   }
 
+  /**
+   * @returns {string}  True if all child nodes are empty
+   */
+  public get isEmpty(): boolean {
+    for (const child of this.childNodes) {
+      if (!child.isEmpty) return false;
+    }
+    return true;
+  }
+
   /**
    * @returns {number}  The number of children allowed, or Infinity for any number,
    *                   or -1 for when an inferred row is needed for the children.
@@ -1023,6 +1034,18 @@ export abstract class AbstractMmlTokenNode extends AbstractMmlNode {
     return true;
   }
 
+  /**
+   * @override
+   */
+  public get isEmpty() {
+    for (const child of this.childNodes) {
+      if (!(child instanceof TextNode) || child.getText().length) {
+        return false;
+      }
+    }
+    return true;
+  }
+
   /**
    * Get the text of the token node (skipping mglyphs, and combining
    *   multiple text nodes)
@@ -1228,6 +1251,13 @@ export abstract class AbstractMmlEmptyNode
     return false;
   }
 
+  /**
+   * @returns {boolean}   Is empty
+   */
+  public get isEmpty(): boolean {
+    return true;
+  }
+
   /**
    * @returns {boolean}  Not embellished
    */

ts/input/tex/base/BaseItems.ts

@@ -788,7 +788,7 @@ export class FnItem extends BaseItem {
         }
       }
       // @test Mathop Apply, Mathop No Apply
-      if (top.isKind('TeXAtom') && top.childNodes[0].childNodes.length === 0) {
+      if (top.isKind('TeXAtom') && top.isEmpty) {
         return [[top, item], true];
       }
       // @test Named Function, Named Function Arg

ts/input/tex/mathtools/MathtoolsUtil.ts

@@ -158,7 +158,7 @@ export const MathtoolsUtil = {
       parser.stack.env,
       parser.configuration
     ).mml();
-    return mml.isKind('TeXAtom') && mml.childNodes[0].childNodes.length === 0
+    return mml.isKind('TeXAtom') && mml.isEmpty
       ? parser.create('node', 'none')
       : mml;
   },