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+import find from "../../polyfills/find.mjs";
+import objectEntries from "../../polyfills/objectEntries.mjs";
+import inspect from "../../jsutils/inspect.mjs";
+import { GraphQLError } from "../../error/GraphQLError.mjs";
+import { Kind } from "../../language/kinds.mjs";
+import { print } from "../../language/printer.mjs";
+import { getNamedType, isNonNullType, isLeafType, isObjectType, isListType, isInterfaceType } from "../../type/definition.mjs";
+import { typeFromAST } from "../../utilities/typeFromAST.mjs";
+
+function reasonMessage(reason) {
+  if (Array.isArray(reason)) {
+    return reason.map(function (_ref) {
+      var responseName = _ref[0],
+          subReason = _ref[1];
+      return "subfields \"".concat(responseName, "\" conflict because ") + reasonMessage(subReason);
+    }).join(' and ');
+  }
+
+  return reason;
+}
+/**
+ * Overlapping fields can be merged
+ *
+ * A selection set is only valid if all fields (including spreading any
+ * fragments) either correspond to distinct response names or can be merged
+ * without ambiguity.
+ */
+
+
+export function OverlappingFieldsCanBeMergedRule(context) {
+  // A memoization for when two fragments are compared "between" each other for
+  // conflicts. Two fragments may be compared many times, so memoizing this can
+  // dramatically improve the performance of this validator.
+  var comparedFragmentPairs = new PairSet(); // A cache for the "field map" and list of fragment names found in any given
+  // selection set. Selection sets may be asked for this information multiple
+  // times, so this improves the performance of this validator.
+
+  var cachedFieldsAndFragmentNames = new Map();
+  return {
+    SelectionSet: function SelectionSet(selectionSet) {
+      var conflicts = findConflictsWithinSelectionSet(context, cachedFieldsAndFragmentNames, comparedFragmentPairs, context.getParentType(), selectionSet);
+
+      for (var _i2 = 0; _i2 < conflicts.length; _i2++) {
+        var _ref3 = conflicts[_i2];
+        var _ref2$ = _ref3[0];
+        var responseName = _ref2$[0];
+        var reason = _ref2$[1];
+        var fields1 = _ref3[1];
+        var fields2 = _ref3[2];
+        var reasonMsg = reasonMessage(reason);
+        context.reportError(new GraphQLError("Fields \"".concat(responseName, "\" conflict because ").concat(reasonMsg, ". Use different aliases on the fields to fetch both if this was intentional."), fields1.concat(fields2)));
+      }
+    }
+  };
+}
+
+/**
+ * Algorithm:
+ *
+ * Conflicts occur when two fields exist in a query which will produce the same
+ * response name, but represent differing values, thus creating a conflict.
+ * The algorithm below finds all conflicts via making a series of comparisons
+ * between fields. In order to compare as few fields as possible, this makes
+ * a series of comparisons "within" sets of fields and "between" sets of fields.
+ *
+ * Given any selection set, a collection produces both a set of fields by
+ * also including all inline fragments, as well as a list of fragments
+ * referenced by fragment spreads.
+ *
+ * A) Each selection set represented in the document first compares "within" its
+ * collected set of fields, finding any conflicts between every pair of
+ * overlapping fields.
+ * Note: This is the *only time* that a the fields "within" a set are compared
+ * to each other. After this only fields "between" sets are compared.
+ *
+ * B) Also, if any fragment is referenced in a selection set, then a
+ * comparison is made "between" the original set of fields and the
+ * referenced fragment.
+ *
+ * C) Also, if multiple fragments are referenced, then comparisons
+ * are made "between" each referenced fragment.
+ *
+ * D) When comparing "between" a set of fields and a referenced fragment, first
+ * a comparison is made between each field in the original set of fields and
+ * each field in the the referenced set of fields.
+ *
+ * E) Also, if any fragment is referenced in the referenced selection set,
+ * then a comparison is made "between" the original set of fields and the
+ * referenced fragment (recursively referring to step D).
+ *
+ * F) When comparing "between" two fragments, first a comparison is made between
+ * each field in the first referenced set of fields and each field in the the
+ * second referenced set of fields.
+ *
+ * G) Also, any fragments referenced by the first must be compared to the
+ * second, and any fragments referenced by the second must be compared to the
+ * first (recursively referring to step F).
+ *
+ * H) When comparing two fields, if both have selection sets, then a comparison
+ * is made "between" both selection sets, first comparing the set of fields in
+ * the first selection set with the set of fields in the second.
+ *
+ * I) Also, if any fragment is referenced in either selection set, then a
+ * comparison is made "between" the other set of fields and the
+ * referenced fragment.
+ *
+ * J) Also, if two fragments are referenced in both selection sets, then a
+ * comparison is made "between" the two fragments.
+ *
+ */
+// Find all conflicts found "within" a selection set, including those found
+// via spreading in fragments. Called when visiting each SelectionSet in the
+// GraphQL Document.
+function findConflictsWithinSelectionSet(context, cachedFieldsAndFragmentNames, comparedFragmentPairs, parentType, selectionSet) {
+  var conflicts = [];
+
+  var _getFieldsAndFragment = getFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, parentType, selectionSet),
+      fieldMap = _getFieldsAndFragment[0],
+      fragmentNames = _getFieldsAndFragment[1]; // (A) Find find all conflicts "within" the fields of this selection set.
+  // Note: this is the *only place* `collectConflictsWithin` is called.
+
+
+  collectConflictsWithin(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, fieldMap);
+
+  if (fragmentNames.length !== 0) {
+    // (B) Then collect conflicts between these fields and those represented by
+    // each spread fragment name found.
+    for (var i = 0; i < fragmentNames.length; i++) {
+      collectConflictsBetweenFieldsAndFragment(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, false, fieldMap, fragmentNames[i]); // (C) Then compare this fragment with all other fragments found in this
+      // selection set to collect conflicts between fragments spread together.
+      // This compares each item in the list of fragment names to every other
+      // item in that same list (except for itself).
+
+      for (var j = i + 1; j < fragmentNames.length; j++) {
+        collectConflictsBetweenFragments(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, false, fragmentNames[i], fragmentNames[j]);
+      }
+    }
+  }
+
+  return conflicts;
+} // Collect all conflicts found between a set of fields and a fragment reference
+// including via spreading in any nested fragments.
+
+
+function collectConflictsBetweenFieldsAndFragment(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fieldMap, fragmentName) {
+  var fragment = context.getFragment(fragmentName);
+
+  if (!fragment) {
+    return;
+  }
+
+  var _getReferencedFieldsA = getReferencedFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, fragment),
+      fieldMap2 = _getReferencedFieldsA[0],
+      fragmentNames2 = _getReferencedFieldsA[1]; // Do not compare a fragment's fieldMap to itself.
+
+
+  if (fieldMap === fieldMap2) {
+    return;
+  } // (D) First collect any conflicts between the provided collection of fields
+  // and the collection of fields represented by the given fragment.
+
+
+  collectConflictsBetween(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fieldMap, fieldMap2); // (E) Then collect any conflicts between the provided collection of fields
+  // and any fragment names found in the given fragment.
+
+  for (var i = 0; i < fragmentNames2.length; i++) {
+    collectConflictsBetweenFieldsAndFragment(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fieldMap, fragmentNames2[i]);
+  }
+} // Collect all conflicts found between two fragments, including via spreading in
+// any nested fragments.
+
+
+function collectConflictsBetweenFragments(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fragmentName1, fragmentName2) {
+  // No need to compare a fragment to itself.
+  if (fragmentName1 === fragmentName2) {
+    return;
+  } // Memoize so two fragments are not compared for conflicts more than once.
+
+
+  if (comparedFragmentPairs.has(fragmentName1, fragmentName2, areMutuallyExclusive)) {
+    return;
+  }
+
+  comparedFragmentPairs.add(fragmentName1, fragmentName2, areMutuallyExclusive);
+  var fragment1 = context.getFragment(fragmentName1);
+  var fragment2 = context.getFragment(fragmentName2);
+
+  if (!fragment1 || !fragment2) {
+    return;
+  }
+
+  var _getReferencedFieldsA2 = getReferencedFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, fragment1),
+      fieldMap1 = _getReferencedFieldsA2[0],
+      fragmentNames1 = _getReferencedFieldsA2[1];
+
+  var _getReferencedFieldsA3 = getReferencedFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, fragment2),
+      fieldMap2 = _getReferencedFieldsA3[0],
+      fragmentNames2 = _getReferencedFieldsA3[1]; // (F) First, collect all conflicts between these two collections of fields
+  // (not including any nested fragments).
+
+
+  collectConflictsBetween(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fieldMap1, fieldMap2); // (G) Then collect conflicts between the first fragment and any nested
+  // fragments spread in the second fragment.
+
+  for (var j = 0; j < fragmentNames2.length; j++) {
+    collectConflictsBetweenFragments(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fragmentName1, fragmentNames2[j]);
+  } // (G) Then collect conflicts between the second fragment and any nested
+  // fragments spread in the first fragment.
+
+
+  for (var i = 0; i < fragmentNames1.length; i++) {
+    collectConflictsBetweenFragments(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fragmentNames1[i], fragmentName2);
+  }
+} // Find all conflicts found between two selection sets, including those found
+// via spreading in fragments. Called when determining if conflicts exist
+// between the sub-fields of two overlapping fields.
+
+
+function findConflictsBetweenSubSelectionSets(context, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, parentType1, selectionSet1, parentType2, selectionSet2) {
+  var conflicts = [];
+
+  var _getFieldsAndFragment2 = getFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, parentType1, selectionSet1),
+      fieldMap1 = _getFieldsAndFragment2[0],
+      fragmentNames1 = _getFieldsAndFragment2[1];
+
+  var _getFieldsAndFragment3 = getFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, parentType2, selectionSet2),
+      fieldMap2 = _getFieldsAndFragment3[0],
+      fragmentNames2 = _getFieldsAndFragment3[1]; // (H) First, collect all conflicts between these two collections of field.
+
+
+  collectConflictsBetween(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fieldMap1, fieldMap2); // (I) Then collect conflicts between the first collection of fields and
+  // those referenced by each fragment name associated with the second.
+
+  if (fragmentNames2.length !== 0) {
+    for (var j = 0; j < fragmentNames2.length; j++) {
+      collectConflictsBetweenFieldsAndFragment(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fieldMap1, fragmentNames2[j]);
+    }
+  } // (I) Then collect conflicts between the second collection of fields and
+  // those referenced by each fragment name associated with the first.
+
+
+  if (fragmentNames1.length !== 0) {
+    for (var i = 0; i < fragmentNames1.length; i++) {
+      collectConflictsBetweenFieldsAndFragment(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fieldMap2, fragmentNames1[i]);
+    }
+  } // (J) Also collect conflicts between any fragment names by the first and
+  // fragment names by the second. This compares each item in the first set of
+  // names to each item in the second set of names.
+
+
+  for (var _i3 = 0; _i3 < fragmentNames1.length; _i3++) {
+    for (var _j = 0; _j < fragmentNames2.length; _j++) {
+      collectConflictsBetweenFragments(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, fragmentNames1[_i3], fragmentNames2[_j]);
+    }
+  }
+
+  return conflicts;
+} // Collect all Conflicts "within" one collection of fields.
+
+
+function collectConflictsWithin(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, fieldMap) {
+  // A field map is a keyed collection, where each key represents a response
+  // name and the value at that key is a list of all fields which provide that
+  // response name. For every response name, if there are multiple fields, they
+  // must be compared to find a potential conflict.
+  for (var _i5 = 0, _objectEntries2 = objectEntries(fieldMap); _i5 < _objectEntries2.length; _i5++) {
+    var _ref5 = _objectEntries2[_i5];
+    var responseName = _ref5[0];
+    var fields = _ref5[1];
+
+    // This compares every field in the list to every other field in this list
+    // (except to itself). If the list only has one item, nothing needs to
+    // be compared.
+    if (fields.length > 1) {
+      for (var i = 0; i < fields.length; i++) {
+        for (var j = i + 1; j < fields.length; j++) {
+          var conflict = findConflict(context, cachedFieldsAndFragmentNames, comparedFragmentPairs, false, // within one collection is never mutually exclusive
+          responseName, fields[i], fields[j]);
+
+          if (conflict) {
+            conflicts.push(conflict);
+          }
+        }
+      }
+    }
+  }
+} // Collect all Conflicts between two collections of fields. This is similar to,
+// but different from the `collectConflictsWithin` function above. This check
+// assumes that `collectConflictsWithin` has already been called on each
+// provided collection of fields. This is true because this validator traverses
+// each individual selection set.
+
+
+function collectConflictsBetween(context, conflicts, cachedFieldsAndFragmentNames, comparedFragmentPairs, parentFieldsAreMutuallyExclusive, fieldMap1, fieldMap2) {
+  // A field map is a keyed collection, where each key represents a response
+  // name and the value at that key is a list of all fields which provide that
+  // response name. For any response name which appears in both provided field
+  // maps, each field from the first field map must be compared to every field
+  // in the second field map to find potential conflicts.
+  for (var _i7 = 0, _Object$keys2 = Object.keys(fieldMap1); _i7 < _Object$keys2.length; _i7++) {
+    var responseName = _Object$keys2[_i7];
+    var fields2 = fieldMap2[responseName];
+
+    if (fields2) {
+      var fields1 = fieldMap1[responseName];
+
+      for (var i = 0; i < fields1.length; i++) {
+        for (var j = 0; j < fields2.length; j++) {
+          var conflict = findConflict(context, cachedFieldsAndFragmentNames, comparedFragmentPairs, parentFieldsAreMutuallyExclusive, responseName, fields1[i], fields2[j]);
+
+          if (conflict) {
+            conflicts.push(conflict);
+          }
+        }
+      }
+    }
+  }
+} // Determines if there is a conflict between two particular fields, including
+// comparing their sub-fields.
+
+
+function findConflict(context, cachedFieldsAndFragmentNames, comparedFragmentPairs, parentFieldsAreMutuallyExclusive, responseName, field1, field2) {
+  var parentType1 = field1[0],
+      node1 = field1[1],
+      def1 = field1[2];
+  var parentType2 = field2[0],
+      node2 = field2[1],
+      def2 = field2[2]; // If it is known that two fields could not possibly apply at the same
+  // time, due to the parent types, then it is safe to permit them to diverge
+  // in aliased field or arguments used as they will not present any ambiguity
+  // by differing.
+  // It is known that two parent types could never overlap if they are
+  // different Object types. Interface or Union types might overlap - if not
+  // in the current state of the schema, then perhaps in some future version,
+  // thus may not safely diverge.
+
+  var areMutuallyExclusive = parentFieldsAreMutuallyExclusive || parentType1 !== parentType2 && isObjectType(parentType1) && isObjectType(parentType2);
+
+  if (!areMutuallyExclusive) {
+    var _node1$arguments, _node2$arguments;
+
+    // Two aliases must refer to the same field.
+    var name1 = node1.name.value;
+    var name2 = node2.name.value;
+
+    if (name1 !== name2) {
+      return [[responseName, "\"".concat(name1, "\" and \"").concat(name2, "\" are different fields")], [node1], [node2]];
+    } // istanbul ignore next (See: 'https://github.com/graphql/graphql-js/issues/2203')
+
+
+    var args1 = (_node1$arguments = node1.arguments) !== null && _node1$arguments !== void 0 ? _node1$arguments : []; // istanbul ignore next (See: 'https://github.com/graphql/graphql-js/issues/2203')
+
+    var args2 = (_node2$arguments = node2.arguments) !== null && _node2$arguments !== void 0 ? _node2$arguments : []; // Two field calls must have the same arguments.
+
+    if (!sameArguments(args1, args2)) {
+      return [[responseName, 'they have differing arguments'], [node1], [node2]];
+    }
+  } // The return type for each field.
+
+
+  var type1 = def1 === null || def1 === void 0 ? void 0 : def1.type;
+  var type2 = def2 === null || def2 === void 0 ? void 0 : def2.type;
+
+  if (type1 && type2 && doTypesConflict(type1, type2)) {
+    return [[responseName, "they return conflicting types \"".concat(inspect(type1), "\" and \"").concat(inspect(type2), "\"")], [node1], [node2]];
+  } // Collect and compare sub-fields. Use the same "visited fragment names" list
+  // for both collections so fields in a fragment reference are never
+  // compared to themselves.
+
+
+  var selectionSet1 = node1.selectionSet;
+  var selectionSet2 = node2.selectionSet;
+
+  if (selectionSet1 && selectionSet2) {
+    var conflicts = findConflictsBetweenSubSelectionSets(context, cachedFieldsAndFragmentNames, comparedFragmentPairs, areMutuallyExclusive, getNamedType(type1), selectionSet1, getNamedType(type2), selectionSet2);
+    return subfieldConflicts(conflicts, responseName, node1, node2);
+  }
+}
+
+function sameArguments(arguments1, arguments2) {
+  if (arguments1.length !== arguments2.length) {
+    return false;
+  }
+
+  return arguments1.every(function (argument1) {
+    var argument2 = find(arguments2, function (argument) {
+      return argument.name.value === argument1.name.value;
+    });
+
+    if (!argument2) {
+      return false;
+    }
+
+    return sameValue(argument1.value, argument2.value);
+  });
+}
+
+function sameValue(value1, value2) {
+  return print(value1) === print(value2);
+} // Two types conflict if both types could not apply to a value simultaneously.
+// Composite types are ignored as their individual field types will be compared
+// later recursively. However List and Non-Null types must match.
+
+
+function doTypesConflict(type1, type2) {
+  if (isListType(type1)) {
+    return isListType(type2) ? doTypesConflict(type1.ofType, type2.ofType) : true;
+  }
+
+  if (isListType(type2)) {
+    return true;
+  }
+
+  if (isNonNullType(type1)) {
+    return isNonNullType(type2) ? doTypesConflict(type1.ofType, type2.ofType) : true;
+  }
+
+  if (isNonNullType(type2)) {
+    return true;
+  }
+
+  if (isLeafType(type1) || isLeafType(type2)) {
+    return type1 !== type2;
+  }
+
+  return false;
+} // Given a selection set, return the collection of fields (a mapping of response
+// name to field nodes and definitions) as well as a list of fragment names
+// referenced via fragment spreads.
+
+
+function getFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, parentType, selectionSet) {
+  var cached = cachedFieldsAndFragmentNames.get(selectionSet);
+
+  if (!cached) {
+    var nodeAndDefs = Object.create(null);
+    var fragmentNames = Object.create(null);
+
+    _collectFieldsAndFragmentNames(context, parentType, selectionSet, nodeAndDefs, fragmentNames);
+
+    cached = [nodeAndDefs, Object.keys(fragmentNames)];
+    cachedFieldsAndFragmentNames.set(selectionSet, cached);
+  }
+
+  return cached;
+} // Given a reference to a fragment, return the represented collection of fields
+// as well as a list of nested fragment names referenced via fragment spreads.
+
+
+function getReferencedFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, fragment) {
+  // Short-circuit building a type from the node if possible.
+  var cached = cachedFieldsAndFragmentNames.get(fragment.selectionSet);
+
+  if (cached) {
+    return cached;
+  }
+
+  var fragmentType = typeFromAST(context.getSchema(), fragment.typeCondition);
+  return getFieldsAndFragmentNames(context, cachedFieldsAndFragmentNames, fragmentType, fragment.selectionSet);
+}
+
+function _collectFieldsAndFragmentNames(context, parentType, selectionSet, nodeAndDefs, fragmentNames) {
+  for (var _i9 = 0, _selectionSet$selecti2 = selectionSet.selections; _i9 < _selectionSet$selecti2.length; _i9++) {
+    var selection = _selectionSet$selecti2[_i9];
+
+    switch (selection.kind) {
+      case Kind.FIELD:
+        {
+          var fieldName = selection.name.value;
+          var fieldDef = void 0;
+
+          if (isObjectType(parentType) || isInterfaceType(parentType)) {
+            fieldDef = parentType.getFields()[fieldName];
+          }
+
+          var responseName = selection.alias ? selection.alias.value : fieldName;
+
+          if (!nodeAndDefs[responseName]) {
+            nodeAndDefs[responseName] = [];
+          }
+
+          nodeAndDefs[responseName].push([parentType, selection, fieldDef]);
+          break;
+        }
+
+      case Kind.FRAGMENT_SPREAD:
+        fragmentNames[selection.name.value] = true;
+        break;
+
+      case Kind.INLINE_FRAGMENT:
+        {
+          var typeCondition = selection.typeCondition;
+          var inlineFragmentType = typeCondition ? typeFromAST(context.getSchema(), typeCondition) : parentType;
+
+          _collectFieldsAndFragmentNames(context, inlineFragmentType, selection.selectionSet, nodeAndDefs, fragmentNames);
+
+          break;
+        }
+    }
+  }
+} // Given a series of Conflicts which occurred between two sub-fields, generate
+// a single Conflict.
+
+
+function subfieldConflicts(conflicts, responseName, node1, node2) {
+  if (conflicts.length > 0) {
+    return [[responseName, conflicts.map(function (_ref6) {
+      var reason = _ref6[0];
+      return reason;
+    })], conflicts.reduce(function (allFields, _ref7) {
+      var fields1 = _ref7[1];
+      return allFields.concat(fields1);
+    }, [node1]), conflicts.reduce(function (allFields, _ref8) {
+      var fields2 = _ref8[2];
+      return allFields.concat(fields2);
+    }, [node2])];
+  }
+}
+/**
+ * A way to keep track of pairs of things when the ordering of the pair does
+ * not matter. We do this by maintaining a sort of double adjacency sets.
+ */
+
+
+var PairSet = /*#__PURE__*/function () {
+  function PairSet() {
+    this._data = Object.create(null);
+  }
+
+  var _proto = PairSet.prototype;
+
+  _proto.has = function has(a, b, areMutuallyExclusive) {
+    var first = this._data[a];
+    var result = first && first[b];
+
+    if (result === undefined) {
+      return false;
+    } // areMutuallyExclusive being false is a superset of being true,
+    // hence if we want to know if this PairSet "has" these two with no
+    // exclusivity, we have to ensure it was added as such.
+
+
+    if (areMutuallyExclusive === false) {
+      return result === false;
+    }
+
+    return true;
+  };
+
+  _proto.add = function add(a, b, areMutuallyExclusive) {
+    this._pairSetAdd(a, b, areMutuallyExclusive);
+
+    this._pairSetAdd(b, a, areMutuallyExclusive);
+  };
+
+  _proto._pairSetAdd = function _pairSetAdd(a, b, areMutuallyExclusive) {
+    var map = this._data[a];
+
+    if (!map) {
+      map = Object.create(null);
+      this._data[a] = map;
+    }
+
+    map[b] = areMutuallyExclusive;
+  };
+
+  return PairSet;
+}();