Series.hs

-- Some edits to the F type

data F :: Timing -> * -> * where
  ...
  {- For each unique `a` value taken on by the first series, call the 
     given function, passing it the `a` wrapped in an `F`, and the 
     subset of issues for each date that have that `a`.  
     
     sector :: F (P Day) String
     -- could very well do something using the k
     By sector (k issues -> Sum issues) (marketCap sp500)      
  -}
  By :: F t a -> (forall s . F s a -> F s b -> F s c) -> F t b -> F t c
  
  -- Another thing we probably need - filtering
  -- Where the first series is > 0, include values from the second series
  -- returns variant data
  Mask :: Num n => F (P t) n -> F (P t) a -> F V a