[Tcepsa]

Not saying there weren't false starts, but your example of eliminate() is actually a fairly common idiom[0] where, when a function is testing things for "truthiness" or "falsiness", it will return the truthy or falsey value rather than actual True or False. It lets code be more brief because you can do e.g. a check for null and perform assignment of either the value or a default in a single statement, so for example something like this:

  if( maybeNullValue ) {
    myValue = maybeNullValue;
  } else {
    myValue = defaultValue;
  }

can instead be expressed as this:

  myValue = maybeNullValue || defaultValue;

I'm personally a little bit torn about it. On the one hand, now that I'm familiar with it I love how much more concise code can be. On the other, it can have a fairly negative impact on code maintainability: if someone comes along who isn't familiar with that idiom, they might have a bad time trying to figure out what's going on (or worse, they might take offense that all these predicates aren't returning True or False values and rewrite the whole mess--or you could end up with a mix of the two, where some functions follow the idiomatic convention and others return actual boolean values which is arguably the worst)

Also I'm not trying to say that there's not some YAGNI going on here--just that I suspect he did it that way somewhat reflexively, and that in that particular case we probably cannot infer he had originally intended to do something else with those results and then changed his mind later.

[0] at least in the Lisp world, which Norvig has extensive experience with[1]

[1] https://github.com/norvig/paip-lisp