Code example on using position hints with multi_type_vector

[Kohei Yoshida]

Hi there,

FYI, I've just added a new code example on multi_type_vector to when to
use position hints:

http://kohei.us/files/mdds/doc/multi_type_vector.html#use-of-position-hint-to-avoid-expensive-block-position-lookup

The concept of position hints may be a bit hard to understand if you are
not used to it, but it *is* used all over Calc core.  So, it's a good
idea to get to know it a little.

Best,

Kohei