Function Description
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	Name: XY1 
	Function: Chemical Shift X+Y <-> XY 
	Parameters: shift, Kd 
		where shift = Dbound - Dfree

Notes
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	This function expects 3 column data.

	Use to fit chemical shift or linebroadening for binding equilibrium
	such as P + L <-> P:L when observing P and varying L. 

		[P][L]/[PL] = kD
		Dobs - Dfree = fbound * (Dbound - Dfree)

C-code for the function
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	 b = x + x2 + Kd;
	 answer = shift * (b - sqrt(b*b - 4*x*x2)) / (2 * x2);