+ * - pass a space-needed for separateDiffBlock and reduce to be able to
+ * reserve more room for diffs, or to force a separate (larger?) block
+ * - Idea: Simplify the structure, so that the first block is always diffs.
+ * On small blocks, that is all we have. Once a block has been merged, we
+ * allocate the first main block and a (new) firstblock ffor diffs. From
+ * that point on the word has two blocks for it.
+ * - On allocating more blocks (in append), check the order of blocks, and
+ * if needed, swap them.
+ * - In merge, merge also with the input data.