Function Description
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	Name: SUM_EXPON_1 
	Function: f(x) = a0 * EXP(-x/a1) + a2 * 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 difficult to obtain convergence. Try holding at least
	one parameter constant before hitting the best fit button.

C-code for the function
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	answer =  a[0] * exp(-x/a[1]) + a[2] * exp(-x/a[3]);