Blocks

 A block is a list of lines within a fold. Blocks may not start in one fold
 and end in another. This is because start and end markers are maintained
 seperately for each fold. Therefore, there may be different marked blocks
 in each fold.

 The block is set by the use of block_mark_1 and block_mark_2 commands and
 is defined to be the lines after the first mark and before the second mark.

 Blocks may be yanked, put copied and deleted just like lines can.

 Blocks may even be written and read from external files, and even passed as
 the stdin of DOS or OS/2 commands, to give new output blocks.