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

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

                    if (parserInput.currentChar() === '$' && (name = parserInput.$re(/^\$[\w-]+/))) {
                        return new(tree.Property)(name, index, fileInfo);
                    }
                }
A Property accessor, such as `$color`, in  background-color: $color