alt Hacker Newsc is an actual input variable, it's not a constant. You can set it to be the constant 1 which the article does in one of the examples, but this isn't mandatory.
c is an actual input variable, it's not a constant. You can set it to be the constant 1 which the article does in one of the examples, but this isn't mandatory.
C is computable from (a, b) regardless.