The answer here has helped me remove single - but not multiple - line breaks. I need to extend this so that it applies only to lines exceeding a specified length, so that: 
This is a longish line whose linebreak should be removed 
so that this line runs together with it.

The preceding double linebreak should be conserved.

This little line
and this one
should stay separated.

[download]
becomes 
This is a longish line whose linebreak should be removed so that this 
+line runs together with it.

The preceding and following double linebreaks should be conserved.

This little line
and this one
should stay separated.

[download]
For example, I'd like to reflow the bodies of email messages while maintaining the formatting of short signatures.

In reply to Single newline removal when line exceeds x characters by tomdyson

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