Implements \mathchoice command (#969)

This PR implements `\mathchoice` function.
I once created PR on the wrong branch. Sorry for the mess.

This is particularly useful when one defines custom macro for "big operators".
For example:

```latex
\newcommand{\infdisj}{%
  \mathop{%
    \mathchoice{% display
      \bigvee\hspace{-2ex}\bigvee%
    }{          % inline
      \bigvee\hspace{-1.75ex}\bigvee%
    }{          % script
      \bigvee\hspace{-1.4ex}\bigvee%
    }{          % scriptscript
      \bigvee\hspace{-1ex}\bigvee%
}}}
```

src/functions.js

@@ -566,3 +566,6 @@ defineFunction(["\\verb"], {
     throw new ParseError(
         "\\verb ended by end of line instead of matching delimiter");
 });
+
+// MathChoice
+import "./functions/mathchoice";

src/functions/mathchoice.js

@@ -0,0 +1,57 @@
+// @flow
+import defineFunction, {ordargument} from "../defineFunction";
+import buildCommon from "../buildCommon";
+import mathMLTree from "../mathMLTree";
+import Style from "../Style";
+import * as html from "../buildHTML";
+import * as mml from "../buildMathML";
+
+const chooseMathStyle = (group, options) => {
+    const style = options.style;
+    if (style.size === Style.DISPLAY.size) {
+        return group.value.display;
+    } else if  (style.size === Style.TEXT.size) {
+        return group.value.text;
+    } else if  (style.size === Style.SCRIPT.size) {
+        return group.value.script;
+    } else if  (style.size === Style.SCRIPTSCRIPT.size) {
+        return group.value.scriptscript;
+    }
+    return group.value.text;
+
+};
+
+defineFunction({
+    type: "mathchoice",
+    names: ["\\mathchoice"],
+    props: {
+        numArgs: 4,
+    },
+    handler: (context, args) => {
+        return {
+            type: "mathchoice",
+            display:      ordargument(args[0]),
+            text:         ordargument(args[1]),
+            script:       ordargument(args[2]),
+            scriptscript: ordargument(args[3]),
+        };
+    },
+    htmlBuilder: (group, options) => {
+        const body = chooseMathStyle(group, options);
+        const elements = html.buildExpression(
+            body,
+            options,
+            false
+        );
+        return new buildCommon.makeFragment(elements);
+    },
+    mathmlBuilder: (group, options) => {
+        const body = chooseMathStyle(group, options);
+        const elements = mml.buildExpression(
+            body,
+            options,
+            false
+        );
+        return new mathMLTree.MathNode("mrow", elements);
+    },
+});

test/__snapshots__/mathml-spec.js.snap

@@ -74,6 +74,149 @@ exports[`A MathML builder should make prime operators into <mo> nodes 1`] = `
 
 `;
 
+exports[`A MathML builder should render mathchoice as if there was nothing 1`] = `
+
+<math>
+  <semantics>
+    <mrow>
+      <mstyle scriptlevel="0"
+              displaystyle="true"
+      >
+        <mrow>
+          <munderover>
+            <mo>
+              ∑
+            </mo>
+            <mrow>
+              <mi>
+                k
+              </mi>
+              <mo>
+                =
+              </mo>
+              <mn>
+                0
+              </mn>
+            </mrow>
+            <mi mathvariant="normal">
+              ∞
+            </mi>
+          </munderover>
+          <msup>
+            <mi>
+              x
+            </mi>
+            <mi>
+              k
+            </mi>
+          </msup>
+        </mrow>
+      </mstyle>
+    </mrow>
+    <annotation encoding="application/x-tex">
+      \\displaystyle\\mathchoice{\\sum_{k = 0}^{\\infty} x^k}{T}{S}{SS}
+    </annotation>
+  </semantics>
+</math>
+
+`;
+
+exports[`A MathML builder should render mathchoice as if there was nothing 2`] = `
+
+<math>
+  <semantics>
+    <mrow>
+      <mrow>
+        <msubsup>
+          <mo>
+            ∑
+          </mo>
+          <mrow>
+            <mi>
+              k
+            </mi>
+            <mo>
+              =
+            </mo>
+            <mn>
+              0
+            </mn>
+          </mrow>
+          <mi mathvariant="normal">
+            ∞
+          </mi>
+        </msubsup>
+        <msup>
+          <mi>
+            x
+          </mi>
+          <mi>
+            k
+          </mi>
+        </msup>
+      </mrow>
+    </mrow>
+    <annotation encoding="application/x-tex">
+      \\mathchoice{D}{\\sum_{k = 0}^{\\infty} x^k}{S}{SS}
+    </annotation>
+  </semantics>
+</math>
+
+`;
+
+exports[`A MathML builder should render mathchoice as if there was nothing 3`] = `
+
+<math>
+  <semantics>
+    <mrow>
+      <msub>
+        <mi>
+          x
+        </mi>
+        <mrow>
+          <mi>
+            T
+          </mi>
+        </mrow>
+      </msub>
+    </mrow>
+    <annotation encoding="application/x-tex">
+      x_{\\mathchoice{D}{T}{\\sum_{k = 0}^{\\infty} x^k}{SS}}
+    </annotation>
+  </semantics>
+</math>
+
+`;
+
+exports[`A MathML builder should render mathchoice as if there was nothing 4`] = `
+
+<math>
+  <semantics>
+    <mrow>
+      <msub>
+        <mi>
+          x
+        </mi>
+        <msub>
+          <mi>
+            y
+          </mi>
+          <mrow>
+            <mi>
+              T
+            </mi>
+          </mrow>
+        </msub>
+      </msub>
+    </mrow>
+    <annotation encoding="application/x-tex">
+      x_{y_{\\mathchoice{D}{T}{S}{\\sum_{k = 0}^{\\infty} x^k}}}
+    </annotation>
+  </semantics>
+</math>
+
+`;
+
 exports[`A MathML builder should use <menclose> for colorbox 1`] = `
 
 <math>

test/katex-spec.js

@@ -2660,3 +2660,31 @@ describe("The maxSize setting", function() {
         expect(built.style.borderTopWidth).toEqual("0em");
     });
 });
+
+describe("The \\mathchoice function", function() {
+    const cmd = "\\sum_{k = 0}^{\\infty} x^k";
+
+    it("should render as if there is nothing other in display math", function() {
+        const plain = getBuilt("\\displaystyle" + cmd)[0];
+        const built = getBuilt(`\\displaystyle\\mathchoice{${cmd}}{T}{S}{SS}`)[0];
+        expect(built).toEqual(plain);
+    });
+
+    it("should render as if there is nothing other in text", function() {
+        const plain = getBuilt(cmd)[0];
+        const built = getBuilt(`\\mathchoice{D}{${cmd}}{S}{SS}`)[0];
+        expect(built).toEqual(plain);
+    });
+
+    it("should render as if there is nothing other in scriptstyle", function() {
+        const plain = getBuilt(`x_{${cmd}}`)[0];
+        const built = getBuilt(`x_{\\mathchoice{D}{T}{${cmd}}{SS}}`)[0];
+        expect(built).toEqual(plain);
+    });
+
+    it("should render  as if there is nothing other in scriptscriptstyle", function() {
+        const plain = getBuilt(`x_{y_{${cmd}}}`)[0];
+        const built = getBuilt(`x_{y_{\\mathchoice{D}{T}{S}{${cmd}}}}`)[0];
+        expect(built).toEqual(plain);
+    });
+});

test/mathml-spec.js

@@ -58,4 +58,16 @@ describe("A MathML builder", function() {
     it('should use <menclose> for colorbox', () => {
         expect(getMathML("\\colorbox{red}{b}")).toMatchSnapshot();
     });
+
+    it('should render mathchoice as if there was nothing', () => {
+        const cmd = "\\sum_{k = 0}^{\\infty} x^k";
+        expect(getMathML(`\\displaystyle\\mathchoice{${cmd}}{T}{S}{SS}`))
+            .toMatchSnapshot();
+        expect(getMathML(`\\mathchoice{D}{${cmd}}{S}{SS}`))
+            .toMatchSnapshot();
+        expect(getMathML(`x_{\\mathchoice{D}{T}{${cmd}}{SS}}`))
+            .toMatchSnapshot();
+        expect(getMathML(`x_{y_{\\mathchoice{D}{T}{S}{${cmd}}}}`))
+            .toMatchSnapshot();
+    });
 });

test/screenshotter/ss_data.yaml

@@ -144,6 +144,8 @@ LowerAccent: |
     \end{matrix}
 MathAtom: a\mathrel{\mathop{=}\limits^{\blue ?}}b
 MathAtom2: \mathop{\overline{\mathrm{lim}}}\limits_{x\to\infty}f(x)
+MathChoice: |
+  {\displaystyle\mathchoice{D}{T}{S}{SS}} {\textstyle\mathchoice{D}{T}{S}{SS}} {\scriptstyle \mathchoice{D}{T}{S}{SS}} {\scriptscriptstyle\mathchoice{D}{T}{S}{SS}} \displaystyle X_{\mathchoice{D}{T}{S}{SS}_{\mathchoice{D}{T}{S}{SS}}}
 MathDefaultFonts: Ax2k\breve{a}\omega\Omega\imath+\KaTeX
 MathBb: \mathbb{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
 MathBf: \mathbf{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}