Company-mode complete custom matches not necessarily prefix

Question

If I type character a, can completion-at-point be used to show possible matches from this list a ɑ æ ɐ ɑ̃, even without matching a prefix?

Answer 1

(defconst sample-completions
  '(("a" "a" "ɑ" "æ" "ɐ" "ɑ̃") ("f" "o" "r")))

(defun company-sample-backend (command &optional arg &rest ignored)
  (interactive (list 'interactive))
  (case command
    (interactive (company-begin-backend 'company-sample-backend))
    (prefix (and (eq major-mode 'fundamental-mode)
         (company-grab-symbol)))
    (candidates

      (dolist (element sample-completions)
        (let ((head (car element)))
          (message "%s -%s" head arg)
           (if (string= head arg)   
               (return element))))
     )))

(add-to-list 'company-backends 'company-sample-backend)

Tweaked from here

Enable M-x company-mode and M-x fundamental-mode, type a or f (the first element of list) and execute company-complete to get completion menu.

How would I enable it only inside quotations?