message: Rename "join" to "merge"
  To avoid the latticey meaning.
diff: @@ -4,7 +4,7 @@ from form_blocks import form_blocks
  import cfg
  -def df_worklist(blocks, bottom, join, transfer):
  +def df_worklist(blocks, bottom, merge, transfer):
  preds, succs = cfg.edges(blocks)
  entry = list(blocks.keys())[0]
  @@ -17,7 +17,7 @@ def df_worklist(blocks, bottom, join, transfer):
  while worklist:
  node = worklist.pop(0)
  - inval = join(outvals[n] for n in preds[node])
  + inval = merge(outvals[n] for n in preds[node])
  invals[node] = inval
  outval = transfer(blocks[node], inval)
  @@ -54,7 +54,7 @@ def run_df(bril):
  df_worklist(
  blocks,
  bottom=set(),
  - join=_union,
  + merge=_union,
  transfer=lambda block, in_: gen(block).union(in_ - kill(block)),
  )
  