public boolean addChildren(List<SpanNode> subtree, double probability) {
        Children childTree = new Children(getParent(), subtree, probability);
        resetCachedFromAndTo();
        return childTrees.add(childTree);

    }
Adds a possible subtree of this AlternateSpanList, with the given probability. Note that the first subtree is
always available through the use of children(), so this method is only used for adding the second or higher
subtree.

@param subtree     the subtree to add
@param probability the probability of this subtree
@return true if successful
@see #children()
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java