Expressions like sin(x) appear unevaluated until a value is substituted for x but expressions like max(x,y) return y! Why is that?

    functions with signatures like (a,b,B(a))->(C(a,b))

    Odd(0)=false, Odd(n)=Even(n-1)
    Even(0)=true, Even(n)=Odd(n-1)