backward-delete-char-untabify is an interactive compiled Lisp function in `simple.el'.

(backward-delete-char-untabify ARG &optional KILLP)

Delete characters backward, changing tabs into spaces.
The exact behavior depends on `backward-delete-char-untabify-method'.
Delete ARG chars, and kill (save in kill ring) if KILLP is non-nil.
Interactively, ARG is the prefix arg (default 1)
and KILLP is t if a prefix arg was specified.