code: @Nullable
    public PSSchema getAsPreprocessedSchema (@Nonnull final PSSchema aSchema) throws SchematronPreprocessException
    {
      ValueEnforcer.notNull (aSchema, "Schema");
  
      // Anything to do?
      if (aSchema.isPreprocessed ())
        return aSchema;
  
      return getForcedPreprocessedSchema (aSchema);
    }
docs: Convert the passed schema to a pre-processed schema.
  
  @param aSchema
  The schema to pre-process. May not be <code>null</code>
  @return The original schema object, if it is already pre-processed - a
  pre-processed copy otherwise! May be <code>null</code> if the
  original schema is not yet pre-processed and
  {@link #isKeepEmptySchema()} is set to <code>false</code>.
  @throws SchematronPreprocessException
  In case a preprocessing error occurs