function sp=sspeed(i,vl,sigma);
%
% returns the shock speed of familiy 'i', shock 'strength' 'sigma'.
%
if i==1,
	sp=0.5*(vl(1)+vl(2))-0.25*exp(-sigma);
elseif i==2,
	sp=0.5*(vl(1)+vl(2))+0.25*exp(-sigma);
else
	error('Wrong i in sspeed');
end;
