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这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

Once upon a time Upas ran on many versions of Unix.
This is a partial rewrite to ANSI C specifically for Plan 9.
It uses's Plan 9's bio library instead of stdio and Plan 9's
regular expression library.

I've tried to make portability possible but it has
never been ported.  To port Upas to another system:

	- port Plan 9's libbio library working on that system (already available).
	- port Plan 9's regexp library working on that system (should just compile).
	- rewrite common/libsys.c to reflect system calls for that system.  This
	  file contains all the really system dependent code that differs between
	  Plan 9 and each Unix.  This includes file management, signal
	  handling, process control and error handling.
	- change the important directory trees in common/mail.c to reflect
	  where you want things like 
	- get the ARGBEGIN/ARGEND/ARGF macros from Plan 9's libc.h
	- get the include files correct in common/sys.h
	- rewrite smtp/mxdial to use the conventions of that system
	- rewrite runq.c to walk queues on that system.