public void setRankType(RankType rankType) {
        this.rankType = rankType;
        for (Index index : getIndices().values()) {
            if (index.getRankType() == null)
                index.setRankType(rankType);
        }

    }
Sets the default rank type of this fields indices, and sets this rank type
to all indices explicitly defined here which has no index set.
(This complex behavior is dues to the fact than we would prefer to have rank types
per field, not per index)
config-model/src/main/java/com/yahoo/schema/document/SDField.java