import the assignment objsets
typo
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complete filter function

What matters is to understand filteracc function and figure out how to
pass acc between left and right. This link triggers an initial thought:

https://class.coursera.org/progfun-004/forum/thread?thread_id=935
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more typo in the given comments
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complete union and mostRetweeted

mostRetweeted with helper function mostRetweetedAcc. reference:
https://class.coursera.org/progfun-004/forum/thread?thread_id=614
create test case for mostRetweeted
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change the most retweeted number to test
complete descendingByRetweet
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modify the default test case
modify descendingByRetweetAcc function

add isEmpty method to the set to allow recursion on this function.
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