'Create block children block linked to current or `parent` if given.
NOTE: Block is added to self.blocks'

def nextblock(self, parent=None):

    block = ControlBlock()
    self.blocks.add(block)
    if parent:
        parent.add_child(block)
    elif self.block:
        self.block.add_child(block)
    self.block = block
    return self.block
