as a fan of S4 i had a look at this; more definitive replies are undoubtedly to come
## now: B00 mother class to B01 and B02, and again B02 "contains" B01 by setIs:
setClass("B00", representation(a="numeric"))
setClass("B01", representation(a="numeric",b="numeric"), contains= "B00")
the direct specification of containment in the next statement is important for the dispatch process
setClass("B02", representation(a="numeric",d="numeric"), contains= "B00")
if we omit the containment assertion your expectations are met
setClass("B02", representation(a="numeric",d="numeric"))
setIs("B02","B01",coerce=function(obj){new("B01", a=obj at a, b=obj at d)},
replace=function(obj,value){new("B01", a=value at a, b=value at b)})
two diagnostic steps that are illuminating are
getClass("B02") under both approaches (with and without contains="B00"
in definition of B02 class)
and
getMethods("+") under both approaches