RefTeX

(add-hook 'yatex-mode-hook 'turn-on-reftex)
(setq reftex-label-alist
    '(AMSTeX
      ("thm" ?h "thm:" "~\\ref{%s}" t ("theorem" "th.") -2)
      ("dfn" ?h "dfn:" "~\\ref{%s}" t ("definition" "def.") -3)))