Getting what came *before* the results of a find call

[Mark Isaacson via Digitalmars-d-learn]

What's the idiomatic way of getting everything *before* the 
results of a call to find?

assert("hello world".find(" world").what_goes_here??? == "hello");

In an article Andrei wrote a few years ago 
(http://www.informit.com/articles/article.aspx…) he mentioned a 
function like this with the name "until", but I can't find it or 
its replacement in Phobos.