code: public final UnicodeSet getSourceSet() {
          UnicodeSet result = new UnicodeSet();
          addSourceTargetSet(getFilterAsUnicodeSet(UnicodeSet.ALL_CODE_POINTS), result, new UnicodeSet());
          return result;
      }
docs: Returns the set of all characters that may be modified in the
  input text by this Transliterator.  This incorporates this
  object's current filter; if the filter is changed, the return
  value of this function will change.  The default implementation
  returns an empty set.  Some subclasses may override {@link
  #handleGetSourceSet} to return a more precise result.  The
  return result is approximate in any case and is intended for
  use by tests, tools, or utilities.
  @see #getTargetSet
  @see #handleGetSourceSet