When does VIM reevaluate foldexpr?

I am trying to figure out when VIM's foldexpr is reevaluated.

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In my case, I have set foldexpr=MySpecialFunction(v:lnum) and it seems to me that MySpecialFunction is only called when the buffer is opened or when I delete a line that closes a fold.

Intuitively, I would have assumed that I can trigger a reevaluation with ctrl-l yet it doesn't.

For what it is worth: my VIM's version is 7.3

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I think, there is no way documented to force reevaluating the foldexpr. I have asked the same some time ago at the vim mailinglist. In the end, a patch was made, that made zx and zX reevaluate the foldexpression, which was good enough for me.

I only noticed two ways to force to reevaluate the foldexpression: to have a noop filter (e.g. filter through cat) or reset the foldexpression.

See the discussion in the vim mailinglist: http://groups.google.com/group/vim_dev/msg/6f13fa0dfcb06317