code: public final IonSystemBuilder withCatalog(IonCatalog catalog)
      {
          IonSystemBuilder b = mutable();
          b.setCatalog(catalog);
          return b;
      }
docs: Declares the catalog to use when building an {@link IonSystem},
  returning a new mutable builder if this is immutable.
  
  @param catalog the catalog to use in built systems.
  If null, each system will be built with a new {@link SimpleCatalog}.
  
  @see #getCatalog()
  @see #setCatalog(IonCatalog)
  @see IonSystem#getCatalog()