Support for equal as a predicate is interesting - there is a discussion going on right now about that in the indy-sdk Hyperledger rocketchat channel.
On the one hand, proving in zero knowledge that a value is equal to a specific value is not really useful from a privacy perspective, because a “true” result means that you already know the value. You can achieve the same result just by just showing the value. That’s the reason that equal may not ever be available as a predicate. You could argue that a “false” result in the predicate means you just know it’s not what you thought, so that is a bit more privacy preserving.
On the other hand, what you may be intending is “prove a credential that has this value in this claim” - that is using the value as a selector to choose on credential from a set of the same credential a holder might have. For example, if you have a number of “Contact” credentials, prove to me the one that has a “type” of “mailing”. If they don’t have a credential with that type, you don’t want the result. That is conveying to the holder what you what in the proof coming back. That’s different from a predicate - that’s a restriction, but that restriction is also not (yet) supported. The team I’m on (BC Gov) is considering adding support for that as a restriction.
It kind of depends on why you are asking for that. Is it just because you wanted to try a predicate and don’t really have a need for that, or some other reason?
One thing we’ve talked about and would like to see in the future, but is not there yet - prove to me that two claims you posses have the same value, but don’t tell me the value. The use case is prove to me that that you are over 18 from driver’s licence credential and that your name in that driver’s licence credential matches the name in the credit card credential you are using to pay for something.
Hope that helps with some context.