public CallStack addAfter(Call after, DocumentProcessor processor) {
        return addAfter(after, new Call(processor, name, metric));
    }
Adds an element just after the first occurence of some other element on
the stack. This can not be called during an iteration.

@param after the call to add this after. If this call is not present, (the
             same object instance), the new processor is added as the last element
@param processor the processor to add
@return this for convenience
docproc/src/main/java/com/yahoo/docproc/CallStack.java