• P = λ( self ).{i = 5, eq = λ(o).(o.i = self.i)}

C = λ( self ).P( self ) \with {b = true,
    eq = λ(o).(o.i = self.i   and
    o.b = self.b)
}
\Y(P) : τ where τ = μα.{i:int, eq:α→ bool}
\Y(C):σ where σ = μα.{i:int, id:α→ bool, b: bool}
However σ\not\leqslant τ
slide: Object inheritance -- contravariance