import { InlineSyntaxNode } from './InlineSyntaxNode'
import { OutlineSyntaxNode } from './OutlineSyntaxNode'
import { InlineSyntaxNodeContainer } from './InlineSyntaxNodeContainer'


export class HeadingNode extends InlineSyntaxNodeContainer implements OutlineSyntaxNode {
  constructor(public children: InlineSyntaxNode[], public level: number) {
    super(children)
  }

  OUTLINE_SYNTAX_NODE(): void { }
}

import { InlineSyntaxNode } from './InlineSyntaxNode'
import { OutlineSyntaxNode } from './OutlineSyntaxNode'
import { InlineSyntaxNodeContainer } from './InlineSyntaxNodeContainer'


export class HeadingNode extends InlineSyntaxNodeContainer implements OutlineSyntaxNode {
  constructor(public children: InlineSyntaxNode[], public level?: number) {
    super(children)
  }

  OUTLINE_SYNTAX_NODE(): void { }
}

--- 
+++ 
@@ -4,7 +4,7 @@
 
 
 export class HeadingNode extends InlineSyntaxNodeContainer implements OutlineSyntaxNode {
-  constructor(public children: InlineSyntaxNode[], public level?: number) {
+  constructor(public children: InlineSyntaxNode[], public level: number) {
     super(children)
   }
 
Make heading constructor require level
