code: public DecomposableMatchBuilder1<T, B> decomposeFirst(DecomposableMatchBuilder0<A> a) {
      Objects.requireNonNull(a);
  
      Map<Integer, DecomposableMatchBuilder> buildersByIndex =
          Maps.treeMapOf(extractedIndexes.first(), a);
  
      List<Matcher<Object>> matchers = getChildMatchers(buildersByIndex);
  
      NestedFieldExtractor<T> nestedFieldExtractor = getNestedFieldExtractor(buildersByIndex);
  
      return new DecomposableMatchBuilder1<>(
          matchers, extractedIndexes.second(), nestedFieldExtractor);
    }
docs: Returns a new {@link DecomposableMatchBuilder1} that further decomposes the first element of
  this match builder.
  
  <p>The first element is decomposed to 0 elements.