In the documentation, class.html describes an invariant function with the following example:

class Date
{
int day;
int hour;

invariant()
{
assert(1 <= day && day <= 31);
assert(0 <= hour && hour < 24);
}
}

I believe (based on experiments with the compiler) that the () need to be left
off the invariant declaration:

invariant
{
assert(1 <= day && day <= 31);
assert(0 <= hour && hour < 24);
}

You're right. I'll make the corrections. Thanks!