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

@param before
           the call to add this before. 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