I have studied N.David Mermin "Relativistic addition of velocities directly from the constancy of the velocity of light," Am.J.Phys. 51 1130 1983 and others with the same subject quoted by the Author. He describes a derivation of the addition law that dispenses not only with the LT but also makes no us of length contraction, time dilation or the relativity of silmultaneity. He starts directly using the concept of speed without mentioning how do we measure it absolute length/absolute time interval; proper length/coordinate time interval or proper length/poper time interval. Does that diminish the value of the derivation?