public void setStyle(int style) {
        if (characters.size() > 1) {
            throw new RuntimeException("Applicable only for single style font");
        }
        Entry<IntMap<BitmapCharacter>> entry = characters.iterator().next();
        IntMap<BitmapCharacter> charset = entry.getValue();
        characters.remove(entry.getKey());
        characters.put(style, charset);
    }
Merge two fonts.
If two font have the same style, merge will fail.
@param styleSet Style must be assigned to this.
author: Yonghoon
jme3-core/src/main/java/com/jme3/font/BitmapCharacterSet.java