docs: public Map<Symbol, Symbol> sourceSymbolMap(int sourceIndex)
      {
          ImmutableMap.Builder<Symbol, Symbol> builder = ImmutableMap.builder();
          for (Map.Entry<Symbol, Collection<Symbol>> entry : outputToInputs.asMap().entrySet()) {
              builder.put(entry.getKey(), Iterables.get(entry.getValue(), sourceIndex));
          }
  
          return builder.build();
      }