code: public DepictionGenerator withAtomNumbers() {
          if (annotateAtomMap || annotateAtomVal)
              throw new IllegalArgumentException("Can not annotated atom numbers, atom values or maps are already annotated");
          DepictionGenerator copy = new DepictionGenerator(this);
          copy.annotateAtomNum = true;
          return copy;
      }
docs: Display atom numbers on the molecule or reaction. The numbers are based on the
  ordering of atoms in the molecule data structure and not a systematic system
  such as IUPAC numbering.
  
  Note: A depiction can not have both atom numbers and atom maps visible
  (but this can be achieved by manually setting the annotation).
  
  @return new generator for method chaining
  @see #withAtomMapNumbers()
  @see StandardGenerator#ANNOTATION_LABEL