While a lambda expression is the most common way of passing a value for a function type, it is not the only option.
If you have a function that matches the signature of the function type (same parameters and same return type), you can use a function reference.
For a top-level function, such as
lessThanZero() in this snippet,
you can refer to the function via a
:: prefix (and leaving off the parentheses),
to pass it as a value to a function type.