Japanese multiplication technique.
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Previously:
https://www.homemadetools.net/forum/...013#post207216
https://www.homemadetools.net/forum/...015#post179894
https://www.homemadetools.net/forum/...1787#post94713
https://www.homemadetools.net/forum/...896#post108875
https://www.homemadetools.net/forum/...166#post115788