  function [a,lambda] = expoFit(tau,y)
% tau and y are column m-vectors with
%         tau(i) < tau(i+1)          , i=1:m
%         y(i)   > y(i+1)            , i=1:m-1
%         y(i)   < (y(i-1)+y(i+1)/2  , i=2:m-1   
%
% lambda is a scalar and a is a column 2-vector with the property that if
%             f(t) = a(1) + a(2)*exp(lambda*t), then
% 
%   (f(tau(1)) - y(1) )^2  + ... + (f(tau(1)) - y(1) )^2
%
% is minimized.

 