[plt-scheme] hash-table-map's order of processing (newb)

In what order does hash-table-map process hash table
entries?  If the answer is "it varies", how and why?  

Where can I find this function's implementation in
MzScheme?  I grep'ed for "hash-table-map" in what got
d/l'ed onto my Mac with DrScheme 370 w/o finding its
definition.  (Though I did see that it was defined for
the very first release of DrScheme back in '95, 0.27.)

thank you,
George Herson