Function Description
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	Name: SUM_EXPON_2 
	Function: f(x) = a0*(1.0 - exp(-x/a1)) + a2*(1.0 - exp(-x/a3))
	Parameters: a0 a1 a2 a3

Notes
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	Fit to a sum of two exponential decay functions
	This function is easy to obtain convergence as opposed to SUM_EXPON_1

C-code for the function
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	answer = a0*(1.0 - exp(-x/a1)) + a2*(1.0 - exp(-x/a3))