Specifying the variance component model "varComp" in R .

[Cynderella]

I am trying to fit a random slope model by "varComp" in R . For the following example , that is in "lmer" syntax , how can I write it in "varComp" syntax :

    library(lme4)
    library(varComp)
    fm1 <- lmer(Reaction ~ Days + (Days||Subject), sleepstudy)

I am not understanding how is to write the random effect part of "varComp" model ?

Many Thanks! Regards.