sym_plus : {x,y:Nat} -> x + y = y + x
if 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` c
Proof of the famous graph theorem about necessary and sufficient condition Eulerian graph: it has exactly two vertices of odd degree.