code: public static Rule getRule(final String expression,
                               final boolean isPostFix) {
      String postFix = expression;
      if (!isPostFix) {
        postFix = CONVERTER.convert(expression);
      }
  
      return new ExpressionRule(COMPILER.compileExpression(postFix));
    }
docs: Get rule.
  @param expression expression.
  @param isPostFix If post-fix.
  @return rule