The circuit computes the 4-to-7 function of seven-segments display.
The picture below, sent to us by one of the solvers, shows a clock built with seven-segments output. The clock has been functioning continuously since 1975.
Gate #20 is wrong. Changing it to NOR provides a solution that is valid for only the decimal digits. If we change it to NXOR we get a valid seven-segments output for all hexadecimal digits.
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