[plt-scheme] what is fold-left?

On Wed, 11 Feb 2009, Mike Eggleston might have said:

> Morning,
> 
> I'm looking at some code at
> <http://funcall.blogspot.com/2007/08/naive-bayesian-classifier.html>. This
> code by default will not run in DrScheme. The code is using (fold-left
> ...). Looking at the function it looks like a (map ...), but I don't
> (yet) know why I can't just use a 'map' instead of the 'fold-left'.
> 
> Any ideas? What does this function do that 'map' doesn't and why
> should it be used?
> 
> Mike

I found a reference here:

<http://docs.plt-scheme.org/r6rs-lib-std/r6rs-lib-Z-H-4.html#node_idx_212>

How to I call this library? I'm using '#lang scheme' as the first
line of my file.

Mike