How to build and install only part of base source tree

[Yasuhiro KIMURA]

Hello,

I would like to add new feature to base system. My idea is simple but
I'm not sure if it can be implemented as is expected.  Therefore I
think some trial and error will be necessary. To test it I need to

(1) Edit some C source files.
(2) Build and install one command and some library files.

And doing buildworld and installworld each time I change source files
is too heavy to repeat.

Then are there any way to build and install only part of base source
tree?

Best Regards.

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Yasuhiro KIMURA