Ease restrictions on when infix base becomes double pipe fenced

fixes #142

Author: runarberg

demo/test-cases.html

@@ -283,6 +283,7 @@ <h2>Super and Subscripts</h2>
         <test-case display="block">(X^T)_ i.,j = X_ j.,i</test-case>
         <test-case display="block">||bf a||^2</test-case>
         <test-case display="block">|bf b|_i</test-case>
+        <test-case display="block">||a*b||^2</test-case>
 
         <test-case display="block">
           ln x = int_1^x 1/t dt

src/compiler/parser/handlers/infix.js

@@ -210,15 +210,16 @@ function rightAssociate(op, [left, right]) {
  * @returns {boolean}
  */
 function isPipeDelimited(nodes) {
-  if (nodes.length !== 3) {
+  if (nodes.length < 3) {
     return false;
   }
 
-  const [open, , close] = nodes;
+  const open = nodes.at(0);
+  const close = nodes.at(-1);
 
   return (
-    open.type === "OperatorLiteral" &&
-    close.type === "OperatorLiteral" &&
+    open?.type === "OperatorLiteral" &&
+    close?.type === "OperatorLiteral" &&
     (open.value === "|" || open.value === "∥" || open.value === "‖") &&
     open.value === close.value
   );

test/snapshots/sub-supscripts.js.md

@@ -99,3 +99,9 @@ Generated by [AVA](https://avajs.dev).
 > Snapshot 1
 
     '<math><msub><mrow><mo>|</mo><mi>𝐛</mi><mo>|</mo></mrow><mi>i</mi></msub></math>'
+
+## Norm of the dot product square
+
+> Snapshot 1
+
+    '<math><msup><mrow><mo>‖</mo><mi>a</mi><mo>·</mo><mi>b</mi><mo>‖</mo></mrow><mn>2</mn></msup></math>'

test/snapshots/sub-supscripts.js.snap

@@ -68,3 +68,7 @@ test("Square of a norm", (t) => {
 test("Index of a magnitute", (t) => {
   t.snapshot(render("|bf b|_i"));
 });
+
+test("Norm of the dot product square", (t) => {
+  t.snapshot(render("||a*b||^2"));
+});