Removing form feed characters

[Murray Cumming <murrayc murrayc com>]

I recently discovered that XML doesn't allow the form feed character
(0xC) in text children (CDATA) even when escaped, though libxml does not
complain about it, and even writes it out to (then invalid) XML.

So I'm thinking about checking for it in Element::set_child_text(),
Element::add_child_text(), ContentNode::set_content() and others. We
could remove the character and maybe warn on stderr.

However, this could cause a processing slowdown, even when people are
not providing a string with that character. Thoughts?

-- 
murrayc murrayc com
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