Function Description
********************

	Name: R2X_DIMER  (R2 Dimer + R2 exchange)
	Function: R2obs = (fM*R2M) + (fD*R2D) + (fM*fM*fD*c)
	Parameters: Kdimer R2dimer R2monomer TAUex DSQU

Notes
*****
	Fit of R2 relaxation data including exchange term to a
	monomer-dimer equilibrium as a function of total protein
	concentration where:

	P + P  <-> P:P; [P][P]/[P:P] = Kdimer
	R2obs = fM*R2M + 2*fD*R2D + fM*fM*fD*TAUex*DSQU 

	Function is undefined at x=0. TAUex and DSQU are treated
	as one term in the numerical analysis. Set one of these constant and let
	the other vary. If the function does not converge with your chosen 
	starting parameters, try a best fit with Kd held constant first.

C-code for the function
***********************

	b = sqrt(Kdimer) * sqrt(Kdimer + 8.0 * x);
	c = 0.25 * (b - Kdimer);
	d = 0.125 * (Kdimer + (4.0 * x) - b);
	e = c / x;
	f = d / x;
	answer = (e * R2monomer) + (2.0 * f * R2dimer) + (e * e * f * TAUex * DSQU);