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var MyFormHandler = Class.create({

  initialize: function(form) {

    this.form = $(form);
    this.index = 0;

    // attach handlers
    this.elements = {};
    this.elements.day = this.form.select('.day')[0];
    this.elements.day.observe('change', this.onDayChanged.bindAsEventListener(this));
  },
  onDayChanged: function(event) {

    // check date value and see if we should focus to the next element
    if (this.elements.day.value.match(/\d+/) && 0 < this.elements.day.value && ... ) {
      this.focusNext();
    }
  },
  focusNext: function() {

    this.index++;
    this.form.getElements()[this.index].focus();
  }

});


// crate a new object for each form (after they exist in the DOM of course)
new MyFormHandler('formId');