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

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

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