escape the TeX rendering of a single underscore as a variable name

A single underscore is a valid variable name.

If the student enters an answer like `a_(b)` and "single-letter variable names" is turned on, it's interpreted as `a * _ * b`.
The underscore wasn't escaped when rendered in LaTeX, so the whole expression would be `a_b`, which looks like it's been interpreted correctly.

This changes `getNameInfo` so a variable name starting with an underscore always counts as `isLong` and is rendered in LaTeX with `\texttt` and the underscores are escaped.

Author: christianp

runtime/scripts/jme.js

@@ -3617,7 +3617,7 @@ var getNameInfo = jme.getNameInfo = function(name) {
         nameInfo.primes = '';
         nameInfo.letterLength = name.length;
     }
-    nameInfo.isLong = nameInfo.letterLength > 1;
+    nameInfo.isLong = nameInfo.letterLength > 1 || nameInfo.root.match(/^_/);
 
     return nameInfo;
 }

tests/jme-runtime.js

@@ -12440,7 +12440,7 @@ var getNameInfo = jme.getNameInfo = function(name) {
         nameInfo.primes = '';
         nameInfo.letterLength = name.length;
     }
-    nameInfo.isLong = nameInfo.letterLength > 1;
+    nameInfo.isLong = nameInfo.letterLength > 1 || nameInfo.root.match(/^_/);
 
     return nameInfo;
 }

tests/jme/jme-tests.mjs

@@ -2321,7 +2321,9 @@ Numbas.queueScript('jme_tests',['qunit','jme','jme-rules','jme-display','jme-cal
             {name: 'x_lambda\'', tex: 'x_{\\lambda}\''},
             {name: 'x_1', tex: '\\dot{x}_{1}', annotations: ['dot'], description: 'annotations only apply to the root, not subscripts'},
             {name: 'ä_1', tex: 'ä_{1}'},
-            {name: 'phi_ß', tex: '\\phi_{ß}'}
+            {name: 'phi_ß', tex: '\\phi_{ß}'},
+            {name: '_', tex: '\\texttt{\\_}'},
+            {name: '_a', tex: '\\texttt{\\_a}'},
         ]
 
         var texifier = new Numbas.jme.display.Texifier();

tests/numbas-runtime.js

@@ -11776,7 +11776,7 @@ var getNameInfo = jme.getNameInfo = function(name) {
         nameInfo.primes = '';
         nameInfo.letterLength = name.length;
     }
-    nameInfo.isLong = nameInfo.letterLength > 1;
+    nameInfo.isLong = nameInfo.letterLength > 1 || nameInfo.root.match(/^_/);
 
     return nameInfo;
 }