purrr:::map 中元素的命名

在这个问题Name output of pmap on tibble中，我询问如何正确命名调用的输出pmap并得到了一个很好的答案，我认为我理解了。

然而，当我进一步研究这种行为时，我不再确定我是否理解其内在原理：

library(purrr)
library(tibble)
params <- tibble(nm = LETTERS[1:2], x = 1:2, y = 2:3, z = 1:2)

## 1. setting names on the first argument gives the expected results
params %>%
  mutate(nm = set_names(nm)) %>% 
  pmap(function(nm, x, y, ...) {
    x + y
  }) %>%
  names()
# [1] "A" "B"


## 2. actually it does not need to be the first element
params %>%
  mutate(nm = set_names(nm)) %>% 
  pmap(function(x, y, nm, ...) {
    x + y
  }) %>%
  names()
# [1] "A" "B"

## 3. however, setting names on `x` does not work
params %>%
  mutate(x = set_names(x, nm)) %>% 
  pmap(function(x, y, nm, ...) {
    x + y
  }) %>%
  names()
# NULL

## 4. I thought maybe because they disappear during the addition `+`, but nope
params %>%
  mutate(z = set_names(z, nm)) %>% 
  pmap(function(x, y, nm, ...) {
    x + y
  }) %>%
  names()
# NULL

## 5. Ok maybe a catch all argument `...` prohibits this behaviour, but no
params %>%
  mutate(z = set_names(z, nm)) %>% 
  pmap(function(x, y, nm, z) {
    x + y
  }) %>%
  names()
# NULL

## 6. It seems that names on a character vector work even if not referenced directly
params %>%
  mutate(nm = set_names(nm)) %>% 
  pmap(function(x, y, ...) {
    x + y
  }) %>%
  names()
# [1] "A" "B"

有人可以告诉我电话中的名字最终是如何确定的吗[p]map？