protected ON DELETE CASCADE

here's my problem

i have tables A, B, C, D

A <- B
A <- C <- D

i want to maintain integrity so that if A is deleted from then so is
anything referencing from B and C - no problem ON DELETE CASCADE

but if there are any D's that point back to A (through composite key in
C) i don't want the delete to go ahead - at all - i want an error
message and condition

can someone who knows the pgsql system well tell me how this is best
done

I've looked at RULE - but how do i delete the old. record (ie confirm
the delete) or conversely prevent the delete?

I've looked at TRIGGER but then i'm writing a function (in SQL?) which
is called by a trigger - again - how do i confirm or reject a delete?

or is it really the case i have to maintain the integrity externally and
write functions that deal with the children and then delete parents -
all in a single transaction - ie not use ON DELETE CASCADE, or rules, or
triggers?

and sorry for cross post but was not sure which was the best list

thanks

murray hobbs
efone.com