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-F = -ma, Isaac Newton

You'll never see a physics textbook introduce Newton's law in this way, but mathematically speaking, it is just as valid as "F = ma". The point of this observation is that if you are careful about what assumptions you turn upside down, you can maintain the validity of your equations, though your result may look surprising. I believed that the model of providing commercial support for open-source software was something that looked impossible because people were so excited about the minus signs that they forgot to count and cancel them.

1989, Cygnus, early open source software company