Using superscript and subscript simultaneously

[emmyoh]

To replicate, try either:

1. \alpha_{2}^{4}
2. \alpha^{4}_{2}

You will observe that they don't render as they should. The subscripts and superscripts should both apply directly to \alpha, not to each other; the two expressions should render identically.

[emmyoh]

The resulting MathML should use the msubsup element.

[ssokolow]

According to the source, it's currently hard-coded to only consider using Node::SubSup in the context of Token::Integral, which is why these are the only two tests using <msubsup>.

(r"\int_0^1 dx",  r#"<msubsup><mo>∫</mo><mn>0</mn><mn>1</mn></msubsup><mi>d</mi><mi>x</mi>"#),
(r"\int^1_0 dx",  r#"<msubsup><mo>∫</mo><mn>0</mn><mn>1</mn></msubsup><mi>d</mi><mi>x</mi>"#),