[plt-scheme] syntax to load an SRFI library in MzScheme

What is the "(require (lib ...))" syntax is to get the SRFI-13 string
library in MzScheme?

I see an SRFI parallel to mzlib, and tried to provide the entire
Unixified path to it, but it's balking.  Also tried testing to see if
SRFI-13 was built-in by trying to access

    (string-contains "abc" "abcdef")

Nope.

The link

   http://practical-scheme.net/wiliki/schemexref.cgi?MzScheme

reports MzScheme has it.

So, how should I incant?

A good link suffices.

Thanks much,

-- Jan