var a = fun (x)
    return x * 2
end

assert a(7) eq 14
assert a(a(7)) eq 28

var b = fun (u, v)
    return u - v
end

assert b(a(2), a(3)) eq -2

var c = fun(x, f)
    return f(f(x))
end

assert c(7, a) eq 28

var d = fun(x)
    var a = fun(x)
        x + 1
    end

    return a(a(x))
end

assert d(3) eq 5