The example in the “A Practical Example” section is quite sloppy about whether we want stocks with prices under 500 or up to 500. This starts with the definition
def isNotOver500(stockPrice : StockPrice) = stockPrice.price < 500
where the name and the condition disagree, and continues on to both the accompanying prose (where “not over 500” and “less than 500” are used interchangeably) and also the following code samples where in
if(isNotOver500(stockPrice)) stockPricesLessThan500 += stockPrice
the two names again disagree.

Granted, this is quite peripheral to the main point of the section, but still a bad example to be given to beginners (who are the target audience of this book).