Cannot cache terminal nodes of inputs to `funnel`

[arendsee]

Given the pipeline:

"a" %v>%
  paste("b") %>%
  funnel("c" %v>% paste("d")) %*>%
  paste %>% plot(label='value')

I can easily cache the nodes wrapping "a", "c", and "a b c d". But I can't find a way to save "b" and "d".

Adding an explicit cacher doesn't work

memcache <- make_recacher(memory_cache)
"a" %v>%
  paste("b") %>% memcache %>%
  funnel("c" %v>% paste("d") %>% memcache) %*>%
  paste %>% plot(label='value')