Add \choose support

Test Plan: Looked at `1 + 2\choose {3 \over 4}`, npm tested.

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D13903

src/functions.js

@@ -164,16 +164,6 @@ var functions = {
                 type: "katex"
             };
         }
-    },
-
-    "\\over": {
-        numArgs: 0,
-        handler: function (func) {
-            return {
-                type: "infix",
-                replaceWith: "\\frac"
-            };
-        }
     }
 };
 
@@ -477,6 +467,31 @@ var duplicatedFunctions = [
                 };
             }
         }
+    },
+
+    // Infix generalized fractions
+    {
+        funcs: ["\\over", "\\choose"],
+        data: {
+            numArgs: 0,
+            handler: function (func) {
+                var replaceWith;
+                switch (func) {
+                    case "\\over":
+                        replaceWith = "\\frac";
+                        break;
+                    case "\\choose":
+                        replaceWith = "\\binom";
+                        break;
+                    default:
+                        throw new Error("Unrecognized infix genfrac command");
+                }
+                return {
+                    type: "infix",
+                    replaceWith: replaceWith
+                };
+            }
+        }
     }
 ];
 

test/katex-spec.js

@@ -559,6 +559,9 @@ describe("An over parser", function() {
     it("should fail with multiple overs in the same group", function () {
         var badMultipleOvers = "1 \\over 2 + 3 \\over 4";
         expect(badMultipleOvers).toNotParse();
+
+        var badOverChoose = "1 \\over 2 \\choose 3";
+        expect(badOverChoose).toNotParse();
     });
 });