code: @Nonnull
    public SchematronResourcePure setVariableResolver (@Nullable final XPathVariableResolver aVariableResolver)
    {
      if (m_aBoundSchema != null)
        throw new IllegalStateException ("Schematron was already bound and can therefore not be altered!");
      m_aVariableResolver = aVariableResolver;
      return this;
    }
docs: Set the variable resolver to be used in the XPath statements. This can only
  be set before the Schematron is bound. If it is already bound an exception
  is thrown to indicate the unnecessity of the call.
  
  @param aVariableResolver
  The variable resolver to set. May be <code>null</code>.
  @return this