Suppose I have an expression like
$A = ( Log[s+t] + s^2 + t ~s + u )$. I want to substitute
$b$ ONLY inside
$Log[s+t]$ without affecting the remaining part. Is there any way to do this? If we directly substitute
$b$ then Mathematica will do the substitution in every places but I want to substitute only in some selected places namely only in the arguments of a function. How can I do that? Hope the problem is clear.