public FilterSpec addExpression(FilterSpec expr){
  if (expressions == null) {
    expressions=new ArrayList<>();
  }
  expressions.add((FilterSpec)expr);
  return this;
}

public FilterSpec addExpression ( FilterSpec expr ) { if ( expressions == null ) { expressions = new ArrayList < > ( ) ; } expressions . add ( ( FilterSpec ) expr ) ; return this ; }
adds the given expression to the expression list and returns itself .
