public CallStack addBefore(Call before, CallStack callStack) {
        int insertPosition = elements.indexOf(before);
        if (insertPosition < 0) {
            addLast(callStack);
        } else {
            elements.addAll(insertPosition, callStack.elements);
        }
        return this;
    }
Adds multiple elements 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 callStack the calls to add
@return this for convenience
docproc/src/main/java/com/yahoo/docproc/CallStack.java