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+digraph shells {

+	size="7,8";

+	node [fontsize=24, shape = plaintext];

+

+	1972 -> 1976;

+	1976 -> 1978;

+	1978 -> 1980;

+	1980 -> 1982;

+	1982 -> 1984;

+	1984 -> 1986;

+	1986 -> 1988;

+	1988 -> 1990;

+	1990 -> future;

+

+	node [fontsize=20, shape = box];

+	{ rank=same;  1976 Mashey Bourne; }

+	{ rank=same;  1978 Formshell csh; }

+	{ rank=same;  1980 esh vsh; }

+	{ rank=same;  1982 ksh "System-V"; }

+	{ rank=same;  1984 v9sh tcsh; }

+	{ rank=same;  1986 "ksh-i"; }

+	{ rank=same;  1988 KornShell Perl rc; }

+	{ rank=same;  1990 tcl Bash; }

+	{ rank=same;  "future" POSIX "ksh-POSIX"; }

+

+	Thompson -> Mashey;

+	Thompson -> Bourne;

+	Thompson -> csh;

+	csh -> tcsh;

+	Bourne -> ksh;

+	Bourne -> esh;

+	Bourne -> vsh;

+	Bourne -> "System-V";

+	Bourne -> v9sh;

+	v9sh -> rc;

+	Bourne -> Bash;

+	"ksh-i" -> Bash;

+	KornShell -> Bash;

+	esh -> ksh;

+	vsh -> ksh;

+	Formshell -> ksh;

+	csh -> ksh;

+	KornShell -> POSIX;

+	"System-V" -> POSIX;

+	ksh -> "ksh-i";

+	"ksh-i" -> KornShell;

+	KornShell -> "ksh-POSIX";

+	Bourne -> Formshell;

+

+	edge [style=invis];

+	1984 -> v9sh -> tcsh ;

+	1988 -> rc -> KornShell;

+	Formshell -> csh;

+	KornShell -> Perl;

+}