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But what is the 'expression' in an array (a b)?

The expression is "a b". It would evaluate to an array containing the two elements.

So null is a different value from false, but both print the same, namely as an empty string?

They print the same if you use var. They do not print the same if you use the # prefix to access a variable's value.

How can I know whether a boolean operator like and returns null or false?

A Boolean operator should return false, not null.

It looks like you were misusing cond. Here is the code you gave me:

set traded cond and match #victim #protected
                      match #mover #protected #mover 0;

The cond operator should have three arguments, but by the time it reaches it, it has none or only one. The match operator is multiple-arity. This means it uses all the arguments to its right. So, whatever value match #mover #protected #mover 0 returns gets included as one of the arguments passed to the other match. Since #mover will match #mover, this value will be true. So, this reduces your expression to:

set traded cond and match #victim #protected true;

Since and receives a single argument, it goes into control flow mode. If #victim matches #protected, we get and true, which allows flow to continue to cond, but cond has no arguments, because and has discarded its argument without returning anything. If it is false, it halts execution of the expression, returning a value of false for the expression. So, you must have gotten a null value for #traded when #victim matched #protected.

I think what you want is this:

set traded cond and == #victim #protected == #mover #protected #mover 0;