Dear Axel, Thanks for your prompt reply. Now I am clear. I will keep in mind
that the variable defined by equal style is more like function.Especially, random() gives different random value whenever it calls although
the arguments x,y and z of random(x,y,z) are all the same. This implies
random() also depends on something such as history of calls…
as it is written in the documentation. the seed (z) is only used
the first time in order to initialize the random number generator.
its purpose is to generate the same sequence for the same seed.
if the seed would be used in _every_ step, there would not be
much randomness. 
axel.