fix single minus sign bug

Author: dqnykamp

lib/trees/default_order.js

@@ -11,7 +11,7 @@ function remove_duplicate_negatives(tree) {
   var operator = tree[0];
   var operands = tree.slice(1);
 
-  if (operator === '-' && operands[0][0] === '-') {
+  if (operator === '-' && Array.isArray(operands[0]) && operands[0][0] === '-') {
     return remove_duplicate_negatives(operands[0][1]);
   }
 
@@ -43,7 +43,7 @@ function normalize_negatives_in_factors(tree) {
   var operands_no_negatives = [];
 
   for (var i = 0; i < operands.length; i++) {
-    if (operands[i][0] === '-') {
+    if (Array.isArray(operands[i]) && operands[i][0] === '-') {
       sign *= -1;
       operands_no_negatives.push(operands[i][1]);
     }
@@ -78,6 +78,9 @@ function sort_key(tree, params = {}) {
   }
   if (typeof tree === 'string') {
     // if string is a constant, return number with value?
+    if(tree === "-" || tree === "+") {
+      return [8, 'plus_minus_string', tree]
+    }
     return [1, 'symbol', tree];
   }
   if (typeof tree === 'boolean') {
@@ -227,7 +230,7 @@ function coeff_factors_from_term(term, string_factors) {
           }
           f[ind]++;
           continue;
-        } else if (Array.isArray(factor) && factor[0] === "^" || factor[0] === '-') {
+        } else if (Array.isArray(factor) && (factor[0] === "^" || factor[0] === '-')) {
           let result = coeff_factors_from_term(factor, string_factors);
           for (let ind in result.factor_contains) {
             if (f[ind] === undefined) {

spec/slow_simplify.spec.js

@@ -563,6 +563,11 @@ describe("collect like terms and factor", function () {
       me.fromText("$(x-b+g) + (y-a+f)% + (z-c+e)deg").default_order().tree)
   })
 
+  it("handle lone - or + signs", function () {
+    expect(me.fromText("cos(2 pi (-))").collect_like_terms_factors().tree).toEqual(me.fromText("cos(2 pi (-))").tree)
+    expect(me.fromText("cos(2 pi (+))").collect_like_terms_factors().tree).toEqual(me.fromText("cos(2 pi (+))").tree)
+  })
+
 });
 
 describe("matrix and vector simplify", function () {