Sieve adding header content to a different header

[@lbutlr]

Given a message itht he headers:

Subject: This is the subject:
X-Foo: bar

how would I do the following:

   1) Add the contents of X-Foo to subject 
      Subject: [bar] This is the subject

   2) replicate the X-Foo header into X-temp
      X-temp: X-foo-bar



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