add another rule for removing brackets involving subtraction

This rule exists to cover a really fiddly case which emerges because of how unary minus is handled inside a product.

When looking for additive terms in the expression `-a - 2*b`, the second term is rewritten so that the unary minus is at the bottom of the tree: `(-2) * b` instead of `-(2*b)`.

This means that the expression ends up looking like `-a + ((-2)*b)`, so it doesn't match the rule `(-?;b - ?;c)`, because the unary minus is too deep.

I've spent a while trying to get the matcher to look deeper for unary minus, but that led to a lot of other things breaking.

In the end, I've added another version of the rule which deals with a `+` inside the brackets instead of a `-`.

Author: christianp

runtime/scripts/jme-rules.js

@@ -2054,6 +2054,7 @@ var simplificationRules = jme.rules.simplificationRules = {
         ['imaginary:$n;z * ?;y `where im(z)<0', 'acsg', '-(eval(-z)*y)'], // Pull negation out of products involving negative imaginary numbers
         ['-(?;a+?`+;b)','','-a-b'],             // Expand negated brackets
         ['?;a+(-?;b-?;c)','','a-b-c'],          // Remove brackets involving subtraction
+        ['?;a+(-?;b+?;c)','','a-b+c'],          // Remove brackets involving subtraction
         ['?;a/?;b/?;c','','a/(b*c)']            // Prefer a product on the denominator to a string of divisions
     ],
     collectComplex: [

tests/jme-runtime.js

@@ -8244,6 +8244,7 @@ var simplificationRules = jme.rules.simplificationRules = {
         ['imaginary:$n;z * ?;y `where im(z)<0', 'acsg', '-(eval(-z)*y)'], // Pull negation out of products involving negative imaginary numbers
         ['-(?;a+?`+;b)','','-a-b'],             // Expand negated brackets
         ['?;a+(-?;b-?;c)','','a-b-c'],          // Remove brackets involving subtraction
+        ['?;a+(-?;b+?;c)','','a-b+c'],          // Remove brackets involving subtraction
         ['?;a/?;b/?;c','','a/(b*c)']            // Prefer a product on the denominator to a string of divisions
     ],
     collectComplex: [

tests/jme/jme-tests.mjs

@@ -2184,6 +2184,14 @@ Numbas.queueScript('jme_tests',['qunit','jme','jme-rules','jme-display','jme-cal
         assert.equal(simplifyExpression('a/(b/c*d)', 'all'), 'a*c/(b*d)', 'a/(b/c*d)');
     });
 
+    QUnit.test('brackets involving subtraction', function(assert) {
+        function simplifyExpression(expr,rules) {
+            return Numbas.jme.display.simplifyExpression(expr,rules || '',Numbas.jme.builtinScope);
+        }
+
+        assert.equal(simplifyExpression('1 + (-a - 2b)'), '1 - a - 2b');
+    });
+
     QUnit.test('localisation doesn\'t affect treeToJME', function(assert) {
         var notation = Numbas.locale.default_number_notation;
         Numbas.locale.default_number_notation = ['plain-eu'];

tests/numbas-runtime.js

@@ -7835,6 +7835,7 @@ var simplificationRules = jme.rules.simplificationRules = {
         ['imaginary:$n;z * ?;y `where im(z)<0', 'acsg', '-(eval(-z)*y)'], // Pull negation out of products involving negative imaginary numbers
         ['-(?;a+?`+;b)','','-a-b'],             // Expand negated brackets
         ['?;a+(-?;b-?;c)','','a-b-c'],          // Remove brackets involving subtraction
+        ['?;a+(-?;b+?;c)','','a-b+c'],          // Remove brackets involving subtraction
         ['?;a/?;b/?;c','','a/(b*c)']            // Prefer a product on the denominator to a string of divisions
     ],
     collectComplex: [