> it is possible to write an amb macro that "chooses" the right argument to make a subsequent assert statement be true.

I'm not too familiar with Scheme's macro feature but how would one go about that? I was under the impression with macros you get to define what ever syntax is part if the call, but here in order to achieve what the author mentions the macro would need to be able to look at surrounding code, to look ahead for the assert. How is this possible? Is he maybe talking of a compiler plugin or a macro (not shown) that encloses the entire expression, e.g. `(make-assert-true-macro (let ...{the code snippet}... ))`?