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