Rotation that points from A to B

f_rotDir()

--BETLOG Hax UTC+10: 20091108 1939 [SLT: 20091108 0239]

You are at vector A and you want to know what rotation (quaternion) will intercept vector B.
Yes it's trivial, but after asking numerous very mathematically intelligent persons, and having them invariably misunderstand what I meant/wanted I feel the need to post it here.
Almost instant understanding and elegant answer courtesy of Nomad Padar.
Dude, I say this a lot, but you are seriously awesome.

The code below is merely a test/example harness for something I cannot post here and would only confuse the solution anyway. So yes, it may appear a little redundant. If you are one of the few people who is pleased to learn how this works by finding this example please comment.
<lsl> //============================================================== vector gTarget; rotation gAngle; //---------------------------------- rotation f_rotDir(vector localAxisToPointWith, vector target, vector origin) { return llRotBetween(localAxisToPointWith*ZERO_ROTATION, target-origin); } //---------------------------------- default { state_entry()

   {   llSensor("B", "", AGENT|PASSIVE|SCRIPTED, 3.0, PI);

// llStopLookAt();

   }
   touch_start(integer num)
   {   if(llDetectedKey(0)==llGetOwner())
       {   llSensor("B", "", AGENT|PASSIVE|SCRIPTED, 3.0, PI);
       }
   }
   sensor(integer num)
   {   vector p=llGetPos();
       gTarget=llDetectedPos(0);

// gAngle=llRotBetween(<1.0, 0.0, 0.0>* <0,0,0,1>, gTarget-p);

       gAngle=f_rotDir(<1.0, 0.0, 0.0>, gTarget, p);
       llRotLookAt(gAngle, 0.1, 0.1);

/* llSetText(""

   +"\n gAngle: "+(string)gAngle
   +"\n SHOULD equal:"
   +"\n llGetRot : "+(string)llGetRot()

, <1.0, 1.0, 1.0>, 1.0);

• /

   }
   no_sensor()
   {   llOwnerSay("B not found!");
       llSetText("B not found!", <1.0, 0.0, 0.0>, 1.0);
   }

} //============================================================== </lsl>