whitespace-mode
whitespace-mode is an interactive autoloaded compiled Lisp function in `whitespace.el
'.
(whitespace-mode &optional ARG)
Toggle whitespace visualization (Whitespace mode).
With a prefix argument ARG, enable Whitespace mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
See also `whitespace-style', `whitespace-newline' and
`whitespace-display-mappings'.