Query super- and subclasses of a class: is there a better way than to use 'completeClassDefinition()'

Dear List,

when I first started to use S4 classes, I used the function
'completeClassDef()' in order to see the super- and subclasses of a
certain class:

setClass(Class="A", representation=list(a="numeric"))
setClass(Class="B", contains="A", representation=list(b="character"))
# Super
x<- completeClassDefinition("B")
attributes(x)
names(x at contains)
# Sub
x<- completeClassDefinition("A")
attributes(x)
names(x at subclasses)

This also does the trick for Reference Classes for me. However, I
re-read the respective section on the help page and wondered if I should
be more careful about using this function for this purpose as the page says:
"|completeClassDefinition: |Completes the definition of |Class|,
relative to the class definitions visible from environment |where|. If
|doExtends| is |TRUE|, complete the super- and sub-class information.
This function is called when a class is defined or re-defined."

So here are my questions:
1) Is it safe to call 'completeClassDef()' explicitly or can anything be
"overwritten" by this?
2) Is there a better way to query the super-/subclasses of a certain
S4/Reference Class?

Best regards and I'd like to take this opportunity to express my
gratitude to everyone on this list who takes the time to provide such
great help!
Janko