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diff --git a/core/misc/states.js b/core/misc/states.js
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+(function ($) {
+
+/**
+ * The base States namespace.
+ *
+ * Having the local states variable allows us to use the States namespace
+ * without having to always declare "Drupal.states".
+ */
+var states = Drupal.states = {
+  // An array of functions that should be postponed.
+  postponed: []
+};
+
+/**
+ * Attaches the states.
+ */
+Drupal.behaviors.states = {
+  attach: function (context, settings) {
+    for (var selector in settings.states) {
+      for (var state in settings.states[selector]) {
+        new states.Dependent({
+          element: $(selector),
+          state: states.State.sanitize(state),
+          dependees: settings.states[selector][state]
+        });
+      }
+    }
+
+    // Execute all postponed functions now.
+    while (states.postponed.length) {
+      (states.postponed.shift())();
+    }
+  }
+};
+
+/**
+ * Object representing an element that depends on other elements.
+ *
+ * @param args
+ *   Object with the following keys (all of which are required):
+ *   - element: A jQuery object of the dependent element
+ *   - state: A State object describing the state that is dependent
+ *   - dependees: An object with dependency specifications. Lists all elements
+ *     that this element depends on.
+ */
+states.Dependent = function (args) {
+  $.extend(this, { values: {}, oldValue: undefined }, args);
+
+  for (var selector in this.dependees) {
+    this.initializeDependee(selector, this.dependees[selector]);
+  }
+};
+
+/**
+ * Comparison functions for comparing the value of an element with the
+ * specification from the dependency settings. If the object type can't be
+ * found in this list, the === operator is used by default.
+ */
+states.Dependent.comparisons = {
+  'RegExp': function (reference, value) {
+    return reference.test(value);
+  },
+  'Function': function (reference, value) {
+    // The "reference" variable is a comparison function.
+    return reference(value);
+  },
+  'Number': function (reference, value) {
+    // If "reference" is a number and "value" is a string, then cast reference
+    // as a string before applying the strict comparison in compare(). Otherwise
+    // numeric keys in the form's #states array fail to match string values
+    // returned from jQuery's val().
+    return (value.constructor.name === 'String') ? compare(String(reference), value) : compare(reference, value);
+  }
+};
+
+states.Dependent.prototype = {
+  /**
+   * Initializes one of the elements this dependent depends on.
+   *
+   * @param selector
+   *   The CSS selector describing the dependee.
+   * @param dependeeStates
+   *   The list of states that have to be monitored for tracking the
+   *   dependee's compliance status.
+   */
+  initializeDependee: function (selector, dependeeStates) {
+    var self = this;
+
+    // Cache for the states of this dependee.
+    self.values[selector] = {};
+
+    $.each(dependeeStates, function (state, value) {
+      state = states.State.sanitize(state);
+
+      // Initialize the value of this state.
+      self.values[selector][state.pristine] = undefined;
+
+      // Monitor state changes of the specified state for this dependee.
+      $(selector).bind('state:' + state, function (e) {
+        var complies = self.compare(value, e.value);
+        self.update(selector, state, complies);
+      });
+
+      // Make sure the event we just bound ourselves to is actually fired.
+      new states.Trigger({ selector: selector, state: state });
+    });
+  },
+
+  /**
+   * Compares a value with a reference value.
+   *
+   * @param reference
+   *   The value used for reference.
+   * @param value
+   *   The value to compare with the reference value.
+   * @return
+   *   true, undefined or false.
+   */
+  compare: function (reference, value) {
+    if (reference.constructor.name in states.Dependent.comparisons) {
+      // Use a custom compare function for certain reference value types.
+      return states.Dependent.comparisons[reference.constructor.name](reference, value);
+    }
+    else {
+      // Do a plain comparison otherwise.
+      return compare(reference, value);
+    }
+  },
+
+  /**
+   * Update the value of a dependee's state.
+   *
+   * @param selector
+   *   CSS selector describing the dependee.
+   * @param state
+   *   A State object describing the dependee's updated state.
+   * @param value
+   *   The new value for the dependee's updated state.
+   */
+  update: function (selector, state, value) {
+    // Only act when the 'new' value is actually new.
+    if (value !== this.values[selector][state.pristine]) {
+      this.values[selector][state.pristine] = value;
+      this.reevaluate();
+    }
+  },
+
+  /**
+   * Triggers change events in case a state changed.
+   */
+  reevaluate: function () {
+    var value = undefined;
+
+    // Merge all individual values to find out whether this dependee complies.
+    for (var selector in this.values) {
+      for (var state in this.values[selector]) {
+        state = states.State.sanitize(state);
+        var complies = this.values[selector][state.pristine];
+        value = ternary(value, invert(complies, state.invert));
+      }
+    }
+
+    // Only invoke a state change event when the value actually changed.
+    if (value !== this.oldValue) {
+      // Store the new value so that we can compare later whether the value
+      // actually changed.
+      this.oldValue = value;
+
+      // Normalize the value to match the normalized state name.
+      value = invert(value, this.state.invert);
+
+      // By adding "trigger: true", we ensure that state changes don't go into
+      // infinite loops.
+      this.element.trigger({ type: 'state:' + this.state, value: value, trigger: true });
+    }
+  }
+};
+
+states.Trigger = function (args) {
+  $.extend(this, args);
+
+  if (this.state in states.Trigger.states) {
+    this.element = $(this.selector);
+
+    // Only call the trigger initializer when it wasn't yet attached to this
+    // element. Otherwise we'd end up with duplicate events.
+    if (!this.element.data('trigger:' + this.state)) {
+      this.initialize();
+    }
+  }
+};
+
+states.Trigger.prototype = {
+  initialize: function () {
+    var self = this;
+    var trigger = states.Trigger.states[this.state];
+
+    if (typeof trigger == 'function') {
+      // We have a custom trigger initialization function.
+      trigger.call(window, this.element);
+    }
+    else {
+      $.each(trigger, function (event, valueFn) {
+        self.defaultTrigger(event, valueFn);
+      });
+    }
+
+    // Mark this trigger as initialized for this element.
+    this.element.data('trigger:' + this.state, true);
+  },
+
+  defaultTrigger: function (event, valueFn) {
+    var self = this;
+    var oldValue = valueFn.call(this.element);
+
+    // Attach the event callback.
+    this.element.bind(event, function (e) {
+      var value = valueFn.call(self.element, e);
+      // Only trigger the event if the value has actually changed.
+      if (oldValue !== value) {
+        self.element.trigger({ type: 'state:' + self.state, value: value, oldValue: oldValue });
+        oldValue = value;
+      }
+    });
+
+    states.postponed.push(function () {
+      // Trigger the event once for initialization purposes.
+      self.element.trigger({ type: 'state:' + self.state, value: oldValue, oldValue: undefined });
+    });
+  }
+};
+
+/**
+ * This list of states contains functions that are used to monitor the state
+ * of an element. Whenever an element depends on the state of another element,
+ * one of these trigger functions is added to the dependee so that the
+ * dependent element can be updated.
+ */
+states.Trigger.states = {
+  // 'empty' describes the state to be monitored
+  empty: {
+    // 'keyup' is the (native DOM) event that we watch for.
+    'keyup': function () {
+      // The function associated to that trigger returns the new value for the
+      // state.
+      return this.val() == '';
+    }
+  },
+
+  checked: {
+    'change': function () {
+      return this.attr('checked');
+    }
+  },
+
+  // For radio buttons, only return the value if the radio button is selected.
+  value: {
+    'keyup': function () {
+      // Radio buttons share the same :input[name="key"] selector.
+      if (this.length > 1) {
+        // Initial checked value of radios is undefined, so we return false.
+        return this.filter(':checked').val() || false;
+      }
+      return this.val();
+    },
+    'change': function () {
+      // Radio buttons share the same :input[name="key"] selector.
+      if (this.length > 1) {
+        // Initial checked value of radios is undefined, so we return false.
+        return this.filter(':checked').val() || false;
+      }
+      return this.val();
+    }
+  },
+
+  collapsed: {
+    'collapsed': function(e) {
+      return (e !== undefined && 'value' in e) ? e.value : this.is('.collapsed');
+    }
+  }
+};
+
+
+/**
+ * A state object is used for describing the state and performing aliasing.
+ */
+states.State = function(state) {
+  // We may need the original unresolved name later.
+  this.pristine = this.name = state;
+
+  // Normalize the state name.
+  while (true) {
+    // Iteratively remove exclamation marks and invert the value.
+    while (this.name.charAt(0) == '!') {
+      this.name = this.name.substring(1);
+      this.invert = !this.invert;
+    }
+
+    // Replace the state with its normalized name.
+    if (this.name in states.State.aliases) {
+      this.name = states.State.aliases[this.name];
+    }
+    else {
+      break;
+    }
+  }
+};
+
+/**
+ * Create a new State object by sanitizing the passed value.
+ */
+states.State.sanitize = function (state) {
+  if (state instanceof states.State) {
+    return state;
+  }
+  else {
+    return new states.State(state);
+  }
+};
+
+/**
+ * This list of aliases is used to normalize states and associates negated names
+ * with their respective inverse state.
+ */
+states.State.aliases = {
+  'enabled': '!disabled',
+  'invisible': '!visible',
+  'invalid': '!valid',
+  'untouched': '!touched',
+  'optional': '!required',
+  'filled': '!empty',
+  'unchecked': '!checked',
+  'irrelevant': '!relevant',
+  'expanded': '!collapsed',
+  'readwrite': '!readonly'
+};
+
+states.State.prototype = {
+  invert: false,
+
+  /**
+   * Ensures that just using the state object returns the name.
+   */
+  toString: function() {
+    return this.name;
+  }
+};
+
+/**
+ * Global state change handlers. These are bound to "document" to cover all
+ * elements whose state changes. Events sent to elements within the page
+ * bubble up to these handlers. We use this system so that themes and modules
+ * can override these state change handlers for particular parts of a page.
+ */
+{
+  $(document).bind('state:disabled', function(e) {
+    // Only act when this change was triggered by a dependency and not by the
+    // element monitoring itself.
+    if (e.trigger) {
+      $(e.target)
+        .attr('disabled', e.value)
+        .filter('.form-element')
+          .closest('.form-item, .form-submit, .form-wrapper')[e.value ? 'addClass' : 'removeClass']('form-disabled');
+
+      // Note: WebKit nightlies don't reflect that change correctly.
+      // See https://bugs.webkit.org/show_bug.cgi?id=23789
+    }
+  });
+
+  $(document).bind('state:required', function(e) {
+    if (e.trigger) {
+      if (e.value) {
+        $(e.target).closest('.form-item, .form-wrapper').find('label').append('<abbr class="form-required" title="' + Drupal.t('This field is required.') + '">*</abbr>');
+      }
+      else {
+        $(e.target).closest('.form-item, .form-wrapper').find('label .form-required').remove();
+      }
+    }
+  });
+
+  $(document).bind('state:visible', function(e) {
+    if (e.trigger) {
+      $(e.target).closest('.form-item, .form-submit, .form-wrapper')[e.value ? 'show' : 'hide']();
+    }
+  });
+
+  $(document).bind('state:checked', function(e) {
+    if (e.trigger) {
+      $(e.target).attr('checked', e.value);
+    }
+  });
+
+  $(document).bind('state:collapsed', function(e) {
+    if (e.trigger) {
+      if ($(e.target).is('.collapsed') !== e.value) {
+        $('> legend a', e.target).click();
+      }
+    }
+  });
+}
+
+/**
+ * These are helper functions implementing addition "operators" and don't
+ * implement any logic that is particular to states.
+ */
+{
+  // Bitwise AND with a third undefined state.
+  function ternary (a, b) {
+    return a === undefined ? b : (b === undefined ? a : a && b);
+  };
+
+  // Inverts a (if it's not undefined) when invert is true.
+  function invert (a, invert) {
+    return (invert && a !== undefined) ? !a : a;
+  };
+
+  // Compares two values while ignoring undefined values.
+  function compare (a, b) {
+    return (a === b) ? (a === undefined ? a : true) : (a === undefined || b === undefined);
+  }
+}
+
+})(jQuery);