--[[--------------------------------------------------------------------
-- nodes for block list (list of active blocks)
-- struct BlockCnt:
--   previous  -- chain (table: BlockCnt)
--   breaklist  -- list of jumps out of this loop
--   nactvar  -- # active local variables outside the breakable structure
--   upval  -- true if some variable in the block is an upvalue (boolean)
--   isbreakable  -- true if 'block' is a loop (boolean)
----------------------------------------------------------------------]]