Fix problem with prescripts being lost if they appear with no base at the end of the expression, and a problem with prescripts when they are before an open brace.
Fix problem with prescripts being lost if they appear with no base at the end of the expression, and a problem with prescripts when they are before an open brace.

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