/usr/share/zsh/functions/Zle/replace-argument is in zsh-common 5.3.1-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 | # Replace an argument to a command, delimited by normal shell syntax.
# Prompts for the replacement.
# With no numeric argument, replace the current argument.
# With a numeric argument, replace that argument: 0 = command word,
# as in history expansion.
# If editing buffer is empty, use previous history line.
autoload -Uz split-shell-arguments read-from-minibuffer
if (( ${#BUFFER} == 0 )); then
  (( HISTNO-- ))
  CURSOR=${#BUFFER}
fi
local widget=$WIDGET numeric
integer cursor=CURSOR
if (( ${+NUMERIC} )); then
  numeric=$NUMERIC
fi
local reply REPLY REPLY2
integer index
split-shell-arguments
if [[ -n $numeric ]]; then
  if (( numeric < 0 )); then
    (( index = ${#reply} - 1 + 2*(numeric+1) ))
  else
    (( index = 2 + 2*numeric ))
  fi
else
  (( index = REPLY & ~1 ))
fi
local edit
if [[ $widget = *edit* ]]; then
  edit=$reply[$index]
fi
read-from-minibuffer "Replace $reply[$index] with: " $edit || return 1
integer diff=$(( ${#REPLY} - ${#reply[$index]} ))
reply[$index]=$REPLY
BUFFER=${(j..)reply}
if (( cursor > REPLY2 )); then
  (( CURSOR = cursor + diff ))
else
  (( CURSOR = REPLY2 ))
fi
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