There is four ways that effects can be defined in GRD. "factor = slope(s)" will vary the means by an amount of s for each step of the factor; "factor = extent(s)" will vary the means uniformly so that there is a difference of s between the first and the last factor level; "factor = custom(a,b,c..)" will alter each means by an amount of a for the first, b for the second, etc. Finally "factor = Rexpression("R code")" will apply R code to all levels of the factors. R code result alters the base mean.
slope(s)
extent(s)
custom(...)
Rexpression(str)the size of the effect
a sequence with the sizes of the effects
R code string
These internal functions are not meant to be used in isolation in any meaningful way...