Scaling in one-direction

I want an object to grow and shrink under the influence of a script, ( touched, timer, etc.)
But llSetScale(<0.1,0,0> grows from the middle in both the positive and negative X directions. What if you want to only grow in the Positive direction?
In Edit, the stretch function has a parameter for one-end only. Does scripting?
Or do I have to resort to a fancy equation, strech both ends by .05 and reposition the center back .05 to grow one end by .1?

You've got the solution in the last part of your post. Shrinking/growing always happens from the center of an object. So after resizing, you'll have to reposition the object. The only way to get around this would be to cut the object in half. Growing/shrinking would still happen in both directions, but only half of it would be visible.

There's a useful example of how to do it at http://wiki.secondlife.com/wiki/Curtain_script

you can do it in 2 calls with
starting with a cube that's <0.25,0.5,0.5> at position <20.0,20.0,20.0> at a zero rotation

vector sizechng = <0.25,0.0,0.0>

llSetScale(llGetScale() + sizechng);//returns <0.5,0.5,0.5>
llSetPos(llGetPos() + (sizechng /2)*llGetLocalRot());//Returns <20.125,20.0,20.0>

basically it moves it half of the amount it changes size so that the negative end stays fixed

you can do it in one call with

llSetPrimitiveParam([PRIM_SIZE,llGetScale() + sizechng,PRIM_POSITION,llGetPos() + (sizechng /2)*llGetLocalRot()]);

something like that, not in world to check it

if you only need it to grow in one direction and never the opposite, you could cut the prim like haruki suggests

to limit x or y growth use either
.125 & .625
or
.375 & .825
to limit z growth, use dimpling
.00 & .50
or
.50 & .00
you can even combine them to get x and z, or y and z limits.

then when you change sizes it will only change in one direction.

Caveat: you have to double your size calculation and your maximum size in that direction becomes 5m for normal prims. z dimples are ignored by flexi's and IIRC sculpts too

Thank you all. Especially Ruthven; this one call version looks particularly good.