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Relations define named rules. 
relation double where
  double(x) = x * 2

query double(5) // 10

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Multiple clauses

Clauses are tried top-to-bottom. The first matching clause wins. 
relation abs where
  abs(x) = x  when x >= 0
  abs(x) = -x when x < 0

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Guards (when)

A clause can have an optional boolean guard: 
relation factorial where
  factorial(0) = 1
  factorial(n) = n * factorial(n - 1) when n > 0