code: public static ImmutableMap<Symbol, Symbol> mapFrom(Map<String, String> stringMap) {
      final ImmutableMap.Builder<Symbol, Symbol> ret = ImmutableMap.builder();
  
      for (Map.Entry<String, String> stringEntry : stringMap.entrySet()) {
        ret.put(Symbol.from(stringEntry.getKey()),
            Symbol.from(stringEntry.getValue()));
      }
  
      return ret.build();
    }
docs: Creates a map of {@link Symbol}s from a map of {@link String}s.  No keys or values may be
  null.