Once again, calling predicates asks questions, which the predicates then answer.  You can ask if multiple things are true using a series of calls; the system will find values for the variables to make all the calls be true simultaneously.  So a series of calls means “and”.  How do we do “or” queries?

The Or predicate

The Or predicate takes a series of calls as parameters, chooses one of them, and runs it:

[Or call1 call2 ...]

It only chooses calls that will succeed (are “true”).  If no call can succeed, then Or fails (is “false”).  And if multiple calls can succeed, it prefers the leftmost one.

The And predicate

The And predicate is the “and” version of the same thing: it takes a series of calls as parameters and runs all of them from left to right:

[Or call1 call2 ...]

If all the calls succeed (are “true”), it succeeds (it's “true”).  If any of them fail, it fails (it's “false”).  Why would you want this if you can do “and” by just making multiple calls?  Because you might want to put an “and” inside an “or”.  This gives you a way of doing that:

[Or [A] [And [B] [C]]]

Will be true (succeed) if either [A] is true or both [B] and [C] are true.