Using core validation within custom validation on CakePHP Forms/Models?

[shiva0101m]

Using CakePHP 1.3 I understand the example at http://book.cakephp.org/complete/1143/Data-Validation#Adding-your-own-Validation-Methods-1181 on how to add my own validation methods, but maybe I'm missing something. I only want to validate that Field_B is 'numeric' (a core validation) if Field_A (a select/option) is equal to a certain value. It seems that I'm supposed to add a custom validation, and then I check the values of other fields with \[code\]$this->data\[/code\], but how do I call on a built-in validation from within a custom validation function? Or am I doing it wrong?