type True = {b};
type False = {a};

type NOT<X> = X extends True ? False : True;

type AND<X,Y> = X extends True ? (Y extends True ? True : False) : False;
type OR<X,Y> = X extends False ? (Y extends False ? False : True) : True;

let x: NOT<OR<NOT<True>,False>>; // x : True

type Num = any[];

type Eval<T extends Num> = T['length'];

type Zero = [];

type Succ<N extends Num> = Prepend<1,N>;

type Prepend<E, T extends any[]> = ((head: E, ...args: T) => any) extends ((...args: infer U) => any) ? U : T;

let y: Eval<Succ<Succ<Zero>>>; // y : 2