[XeTeX] Emulating font extension

[Vadim Radionov]

Hello,
Thank you for the great engine!

I'm wondering what exactly the "extend" pseudo-feature does: if set
unreasonably big (say, 2), the interword spacing shrinks to zero and
even negative, and if it is as small as .5, the space is doubled or
so.  I suppose it changes the spacing for "normal" values of 1.01 or
.99 too, so I would like to know how to correct it by changing the
space by some amount.

I use XeTeX 3.1415926-2.2-0.9994.0 (Web2C 7.5.6)

Thank you in advance
Vadim