checkers pattern by tails

:1
s/\w/&-/g
y/0123456789/+012345678/
s/-+/9/
s/+-//
t1
h
s/.* //
G
s/-- .*//
:
s/-/ 1/
s// 0/
t
s/^ //
P
y/01/10/
s/ .$//
t

Note that non-ascii characters in the above source code will be escaped (such as \x9f).

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