If Conditions (Harlowe)

I've checked the Harlowe manual repeatedly and tried searching through the forums, but been unable to find an answer:

Is there a list somewhere of what conditions are allowed in if statements?

So far I've come across: is, contains, > < >= <= (but not =< or =>)

But I'm wondering if there are any others. If someone could fill in the rest or point me towards the documentation I'm unable to find I'd really appreciate it.