Thank you so much!!
That is very helpful for me.
BTW
Can I add some conditions in <subst>?
The original xhtml file: 
....
 <mml:mi>s</mml:mi>
 <mml:mi>sin</mml:mi>
..

[download]

Only <mml:mi>s</mml:mi> just replace with <mml:mi>x</mml:mi>.
the result:
New xhtml file 
....
 <mml:mi>x</mml:mi>
 <mml:mi>sin</mml:mi>
..

[download]

In reply to Re^2: Replace MathML content using Twig by renovatio
in thread Replace MathML content using Twig by renovatio

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