lib/less/parser/parser.js
function (isLess) {
                var index = parserInput.i, elements, extendList, c, e, allExtends, when, condition;
                isLess = isLess !== false;
                while ((isLess && (extendList = this.extend())) || (isLess && (when = parserInput.$str('when'))) || (e = this.element())) {
                    if (when) {
                        condition = expect(this.conditions, 'expected condition');
                    } else if (condition) {
                        error('CSS guard can only be used at the end of selector');
                    } else if (extendList) {
                        if (allExtends) {
                            allExtends = allExtends.concat(extendList);
                        } else {
                            allExtends = extendList;
                        }
                    } else {
                        if (allExtends) { error('Extend can only be used at the end of selector'); }
                        c = parserInput.currentChar();
                        if (elements) {
                            elements.push(e);
                        } else {
                            elements = [ e ];
                        }
                        e = null;
                    }
                    if (c === '{' || c === '}' || c === ';' || c === ',' || c === ')') {
                        break;
                    }
                }

                if (elements) { return new(tree.Selector)(elements, allExtends, condition, index, fileInfo); }
                if (allExtends) { error('Extend must be used to extend a selector, it cannot be used on its own'); }
            }
function (isLess) {
                var index = parserInput.i, elements, extendList, c, e, allExtends, when, condition;
                isLess = isLess !== false;
                while ((isLess && (extendList = this.extend())) || (isLess && (when = parserInput.$str('when'))) || (e = this.element())) {
                    if (when) {
                        condition = expect(this.conditions, 'expected condition');
                    } else if (condition) {
                        error('CSS guard can only be used at the end of selector');
                    } else if (extendList) {
                        if (allExtends) {
                            allExtends = allExtends.concat(extendList);
                        } else {
                            allExtends = extendList;
                        }
                    } else {
                        if (allExtends) { error('Extend can only be used at the end of selector'); }
                        c = parserInput.currentChar();
                        if (elements) {
                            elements.push(e);
                        } else {
                            elements = [ e ];
                        }
                        e = null;
                    }
                    if (c === '{' || c === '}' || c === ';' || c === ',' || c === ')') {
                        break;
                    }
                }

                if (elements) { return new(tree.Selector)(elements, allExtends, condition, index, fileInfo); }
                if (allExtends) { error('Extend must be used to extend a selector, it cannot be used on its own'); }
            }
A CSS Selector with less extensions e.g. the ability to extend and guard  .class > div + h1 li a:hover  Selectors are made out of one or more Elements, see above.