### Question:
AutoComplete { public AutoComplete union(AutoComplete other) { if (possibilities.isEmpty()) { return other; } if (other.possibilities.isEmpty()) { return this; } if (!this.prefix.equals(other.prefix)) { throw new IllegalArgumentException("Trying to perform union on different prefixes: prefix1='"+this.prefix+"', prefix2='"+other.prefix+'\''); } final Trie<CliValueType> unifiedPossibilities = this.possibilities.union(other.possibilities); return new AutoComplete(prefix, unifiedPossibilities); } AutoComplete(String prefix, Trie<CliValueType> possibilities); Opt<String> getAutoCompleteSuffix(); Opt<Suggestions> getSuggestions(); AutoComplete union(AutoComplete other); @Override String toString();  }### Answer:
@Test public void testUnion() throws Exception { }