--- tapply.R	2016-01-09 12:19:58.188759800 +0700
+++ tapply269890newoldc.R	2016-01-09 19:33:01.734375000 +0700
@@ -37,12 +37,9 @@
         for (i in 2L:nI)
            group <- group + cumextent[i - 1L] * (as.integer(INDEX[[i]]) - 1L)
     if (is.null(FUN)) return(group)
-    levels(group) <- as.character(seq_len(ngroup))
-    class(group) <- "factor"
-    ans <- split(X, group) # use generic, e.g. for 'Date'
+    ans <- lapply(X = split(X, group), FUN = FUN, ...) # use generic, e.g. for 'Date'
+    index <- as.integer(names(ans))
     names(ans) <- NULL
-    index <- as.logical(lengths(ans))  # equivalently, lengths(ans) > 0L
-    ans <- lapply(X = ans[index], FUN = FUN, ...)
     if (simplify && all(lengths(ans) == 1L)) {
 	ansmat <- array(dim = extent, dimnames = namelist)
 	ans <- unlist(ans, recursive = FALSE)