.SH Acknowledgement .PP I am grateful to Ted Dolotta for his careful reading and valuable suggestions. .SH References .IP [1] Brian W. Kernighan, .ul A Tutorial Introduction to the UNIX Text Editor, Bell Laboratories internal memorandum. .IP [2] Brian W. Kernighan, .ul UNIX For Beginners, Bell Laboratories internal memorandum. .IP [3] Ken L. Thompson and Dennis M. Ritchie, .ul The UNIX Programmer's Manual. Bell Laboratories.