I was looking for something similar and came across this question. Then I was happy to find Mathquill through the Wikipedia page for formula editors .
I used a bunch of different formula editors, from the MS Equation editor to Google Docs to LyX, and this is probably the most convenient for all of them to simply beat out formulas. And this is the web interface and GPL. This thing is much better than the Google Docs formula editor, at least.
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