n <- 3.2e6
  DT.mini <- DT.Spotify.Counts.by.user.product.date[{set.seed(1); sampleInChunks(DT.Spotify.Counts.by.user.product.date, n=n)}]


  DT.TEST <- DT.mini[, list(MonthsUsed=.N), keyby=list(customerid, product)]
  Customers.TEST <- DT.TEST[, .N, by=customerid][N>1, customerid]

  setkeyIfNot(DT.mini, "customerid")
# ======================================================= #


  ## OLD: 
  Old <- quote({
    DT.old <- DT.mini[Customers.TEST]
    cleanSpotify_(DT.old)
    DT.old <- DT.old[, list( year=year[-1L], month=month[-1L]
                             , ProductFrom = product[- (.N) ]
                             , ProductTo = product[-1L]
                             , ProductDiff=diff(as.integer(product))
                            )
                      , keyby=customerid]
    colsOrdered <- c("customerid", "year", "month", "ProductFrom", "ProductTo", "ProductDiff")
    setcolorderpt(DT.old, colsOrdered)
  })

  ## NEW: 
  New <- quote(
  {
    DT.new <- DT.mini[Customers.TEST
                         , cbind(.SD[-1], ProductFrom=product[-.N])
                         , .SDcols=c("year", "month", "product")
                         # by-without-by
                         # , keyby=customerid
                         ]
    ## Fix Names
    setnames(DT.new, "product", "ProductTo")
    ## Convert to Factors
    dict.product <- c(Daypass="D", Open="O", Free="F", Limited="L", DesktopBasic="U", Premium="P")
    DT.new [, ProductTo   := factor(ProductTo,   levels=dict.product, labels=names(dict.product))]
    DT.new [, ProductFrom := factor(ProductFrom, levels=dict.product, labels=names(dict.product))]
    ## Take Diff
    DT.new[, ProductDiff := as.numeric(ProductTo) - as.numeric(ProductFrom)]

    colsOrdered <- c("customerid", "year", "month", "ProductFrom", "ProductTo", "ProductDiff")
    setcolorderpt(DT.new, colsOrdered)
  })




a.old <-attributes(DT.old)
a.new <-attributes(DT.new)

if(all ( mapply(identical, a.old, a.new[names(a.old)]) )
  && all ( mapply(identical, a.old[names(a.new)], a.new) )
  && all ( eval(Old) == eval(New) )
  )
  mbench(New, Old, times=5L, check=FALSE)



n = 3.2e5
----------
 Measuring 'elapsed' time.
 Units: milliseconds                          Number of Repetitions: 5

   Expr      relative      Median Time       MAD  
----------|-------------|----------------|---------
   Old           1            19.26         0.09  
   New          12            229.23        0.08  


n = 6e5
----------
 Measuring 'elapsed' time.
 Units: milliseconds                          Number of Repetitions: 5

   Expr      relative      Median Time       MAD  
----------|-------------|----------------|---------
   Old           1            55.76         1.13  
   New          14            767.45        2.51  


n = 3.2e6
---------
Measuring 'elapsed' time.
Units: seconds                                Number of Repetitions: 5

   Expr      relative      Median Time      MAD    
----------|-------------|----------------|---------
   Old           1             1.38         0.04    
   New          26            35.48         0.02    
## NOTE:                   Time is in seconds.