function interp = expinterp(x, x0,x1,y0,y1)
% explicit interpolation of x between x0 and x1, into
% interval y0,y1.  0's always less than 1's.
xd = x1-x0;
yd = y1 - y0;
interp = y0 + (x-x0)*(yd/xd);
end
 