Interacting Dynamic Components within LocatorPane

I am attempting to create a dynamic contour plot of an ellipse with two locators as the foci within a locator pane. The following code has been implemented:

a:= Sqrt[c^2 + b^2]
b = 1;
c = 0;

ellipse := (x^2)/(a^2) + (y^2)/(b^2)
plot := Dynamic[ContourPlot[ellipse == 1, {x,-2,2}, {y,-2,2}]

LocatorPane[Dynamic[{ {c,0}, {-c,0} }], plot]


When I evaluate this, the plot behaves pretty much as desired except that only one of the locators can be moved.

This is because I placed -c inside Dynamic as opposed to just the variable c. Ordinarily, I would just do something like Dynamic[{-c,0}, (c = -#)&], however, LocatorPane requires that I include the full list of locators within a single Dynamic.

I want to find a way to apply a function to Dynamic that causes the first component of the second locator to take on the value of -c while the first locator is using c.

Any help?

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b = 1;
ellipse[x_, y_, pt_] := (x^2)/(pt[[1]]^2 + b^2) + (y^2)/(b^2)
DynamicModule[{pt = {0, 0}},
LocatorPane[Dynamic[pt],
Dynamic@ContourPlot[ellipse[x, y, pt] == 1, {x, -2, 2}, {y, -2, 2},
Epilog -> {PointSize[Large], Point[Dynamic[{First[pt], 0}]],
Point[Dynamic[{-First[pt], 0}]]}], Appearance -> None]]


• Clean! +1. Btw. why Epilog needs Dynamic@Locator in: x = {0, 0}; { Graphics[{Locator@Dynamic@x}], Graphics[{}, Epilog -> {Locator@Dynamic@x}], Dynamic[x] } – Kuba Feb 19 '15 at 15:19
• @Kuba Thanks:)! You need the Dynamic[] to update and the Locator[] to know what to display. You may change the Locator[] by Point[], for example. – Dr. belisarius Feb 19 '15 at 15:23
• @Kuba Oh, I see now. Not sure, let me think (or post a question!) – Dr. belisarius Feb 19 '15 at 16:18
• @Kuba There was a question about "how deep" Dynamic "sees" when it acts on a expression. I believe it is related to that – Dr. belisarius Feb 19 '15 at 16:24
• @belisarius any idea which one? :) – Kuba Feb 20 '15 at 10:29