code: public IonReaderBuilder withCatalog(IonCatalog catalog)
      {
          IonReaderBuilder b = mutable();
          b.setCatalog(catalog);
          return b;
      }
docs: Declares the catalog to use when building an {@link IonReader},
  returning a new mutable builder the current one is immutable.
  
  @param catalog the catalog to use in built readers.
  If null, a new {@link SimpleCatalog} will be used.
  
  @return this builder instance, if mutable;
  otherwise a mutable copy of this builder.
  
  @see #setCatalog(IonCatalog)
  @see #withCatalog(IonCatalog)