Exit if the minibuffer contains a valid completion.
Otherwise, try to complete the minibuffer contents. If
completion leads to a valid completion, a repetition of this
command will exit.
If `minibuffer-completion-confirm' is `confirm', do not try to
complete; instead, ask for confirmation and accept any input if
If `minibuffer-completion-confirm' is `confirm-after-completion',
do not try to complete; instead, ask for confirmation if the
preceding minibuffer command was a member of
`minibuffer-confirm-exit-commands', and accept the input otherwise. (fn)