Prove that every natural number can be written as a sum of distinct non-consecutive Fibonacci numbers. For example, 22=1+3+5+1322=1+3+5+1322 = 1 + 3 + 5 + 13 is not allowed, since 3 and 5 are consecutive Fibonacci numbers, but 22=1+2122=1+2122 = 1 + 21 is allowed.

Lean’s logic is an even more elaborate and expressive system of logic, which fully subsumes all the notions of higher-order logic we have discussed here.

Interpretations that give two or more classes for different quantifiers to range over are said to be many-sorted, and the classes are sometimes called the sorts.

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Whereas normal type classes represent predicates on types (each type is either an instance of a type class or it isn’t), multi-parameter type classes represent relations on types

One hopes, that at some point in the future, the best parts of stack/Stackage and cabal/Hackage can be merged to build a unified, kickass build-tool. Till that day comes, just use Stack.