cmpl-prefix-entry-headcmpl-prefix-entry-head is an alias for `car' in `
completion.el'. (cmpl-prefix-entry-head LIST) Return the car of LIST. If arg is nil, return nil. Error if arg is not nil and not a cons cell. See also `car-safe'. See Info node `(elisp)Cons Cells' for a discussion of related basic Lisp concepts such as car, cdr, cons cell and list.