sym_plus : {x,y:Nat} -> x + y = y + xif b|a and b|c then c|a
on idris:
data Divb : Nat -> Nat -> Type where
DivbAx1 : a = k*b -> a `Divb` b
theorem : a `Divb` b -> b `Divb` c -> a `Divb` cProof of the famous graph theorem about necessary and sufficient condition Eulerian graph: it has exactly two vertices of odd degree.