lib/less/parser/parser.js
function () {
                    var curly, index = parserInput.i;

                    if (parserInput.currentChar() === '@' && (curly = parserInput.$re(/^@\{([\w-]+)\}/))) {
                        return new(tree.Variable)('@' + curly[1], index, fileInfo);
                    }
                }
function () {
                    var curly, index = parserInput.i;

                    if (parserInput.currentChar() === '@' && (curly = parserInput.$re(/^@\{([\w-]+)\}/))) {
                        return new(tree.Variable)('@' + curly[1], index, fileInfo);
                    }
                }
A variable entity using the protective {} e.g. @{var}