Lookup stack container for stretchy operators for msubsup in CHTML.  Resolves issue #1287.
Lookup stack container for stretchy operators for msubsup in CHTML.  Resolves issue #1287.

thomasjm/MathJax,thomasjm/MathJax,GerHobbelt/MathJax,thomasjm/MathJax,jinntrance/mathjax,dpvc/MathJax,wikimedia/MathJax,thomasjm/MathJax,GerHobbelt/MathJax,jinntrance/mathjax,thomasjm/MathJax