public CallStack getCallStack() {
        return callStack;
    }
Creates a new named DocprocExecutor, with the same instance variables as the given executor,
but a new call stack.

@param oldExecutor the executor to inherit the instance variables from, sans call stack.
@param callStack the call stack to use.
docproc/src/main/java/com/yahoo/docproc/impl/DocprocExecutor.java