Narrowing

Following commands seem to help very much.

In short, with "narrow" I leave only current defun in the buffer to edit, with "widen" I show complete file again.

C-x n d ...... Narrow down to the current defun (`narrow-to-defun').
C-x n w ...... Widen to make the entire buffer accessible again (`widen').

There are two more commands for region and page, but they are useless for me.
Anyway in Emacs' help one can find full description in the chapter called "Narrowing".

And yes, I added following three lines to my .emacs file to enable described functionality:

(put 'narrow-to-defun 'disabled nil)
(put 'narrow-to-page 'disabled nil)
(put 'narrow-to-region 'disabled nil)