The model displays two basic conic sections: hyperbolas
and parabolas. The figures are generated behaviorally as 
opposed to algebraically- the turtles attempt to behave 
like points on the specified shape. The partner to this
model is called 'Conic Sections-circle&ellipse.'

The turtles use feedback to make decisions about how they
behave. They set out in random directions, and then they
receive information as to whether or not they are getting
closer to where they want to be. If they are getting closer,
they continue moving forward in the direction they are
going. If they are moving farther away, they set out in a
new random direction. This process is akin to the children's
game of "Hot & Cold", in which players are told whether they
are getting "hotter" or "colder" in relation to a hidden goal.

	-Select the number of turtles with the TURTLES slider.
	-Press SETUP.
	-Make sure the DIRECTRIX switch is set to 1.
	-Press the MOVE-FOCUS-DIRECTRIX button to select the two foci
	 as for ellipses.
	-Press MOVE-TURTLES. Adjust CONSTANT and pick new foci as desired.
	-Select the number of turtles with the TURTLES slider.
	-Press SETUP.
	-Make sure the DIRECTRIX switch is set to 0.
	-Press the MOVE-FOCUS-DIRECTRIX button. After this, select first
	the position of the focus, and then a point through which
	the directrix will pass.  The SLOPE slider sets the slope of the 		
	-Press MOVE-TURTLES. You can use the slope slider to change
	the slope of the directrix.
	-Note that CONSTANT has no effect on the parabola.

When forming a hyperbola, turtles adjust their positions from two
user-defined foci so that the difference between their
distances from the foci attains a value of CONSTANT.

When forming a parabola, turtles move until their distance 
from the directrix, measured perpendicularly, is the same as their 
distance to the focus.

In some cases, the turtles cannot satisfy the rule.  In this case, 
they keep looking and never settle down.  Look for examples of this 
with a hyperbola.  

Adjust the slope of the parabola's directrix or the value
of CONSTANT for the hyperbola while the turtles are still
moving. See how they react to the changes in their

You may be able to get a better feeling for the turtles'
behavior if only a few turtles are on-screen at one time.
Try setting num to a small value (like 16 or 1), and
watching the turtles.

Look at the StarLogoT model Turtles Circling.  Watch how the
turtles react with each other- something that is missing
from 'Conic Sections'. Implement this emergent
behavior for one or all of the conics in this project.

Like more traditional programming languages (e.g. C++),
StarLogoT can have functions that return a value to the
calling function. The command used is called 'output'- it
has one argument, the value to be returned. Look at the
function 'minus'. It takes one argument, num, and returns its
negative, 0 - num. (The colon preceding num indicates that
num is a local variable sent to the called function.)