@Override public Value concatenate(Value value){
  if (!(value instanceof RelationalVal)) {
    throw new RuntimeException("Cannot concatenate " + this + " with "+ value);
  }
  return new RelationalVal(toString() + value.toString());
}

@ Override public Value concatenate ( Value value ) { if ( ! ( value instanceof RelationalVal ) ) { throw new RuntimeException ( "Cannot concatenate " + this + " with " + value ) ; } return new RelationalVal ( toString ( ) + value . toString ( ) ) ; }
concatenates two relational structures ( by juxtaposing their roots ) .
