(function (tree) {

tree.URL = function (val, paths) {
    if (val.data) {
        this.attrs = val;
    } else {
        // Add the base path if the URL is relative and we are in the browser
        if (!/^(?:https?:\/|file:\/)?\//.test(val.value) && paths.length > 0 && typeof(window) !== 'undefined') {
            val.value = paths[0] + (val.value.charAt(0) === '/' ? val.value.slice(1) : val.value);
        }
        this.value = val;
        this.paths = paths;
        this.is = 'uri';
    }
};
tree.URL.prototype = {
    toCSS: function () {
        return "url(" + (this.attrs ? 'data:' + this.attrs.mime + this.attrs.charset + this.attrs.base64 + this.attrs.data
                                    : this.value.toCSS()) + ")";
    },
    eval: function (ctx) {
        return this.attrs ? this : new(tree.URL)(this.value.eval(ctx), this.paths);
    }
};

})(require('mess/tree'));

(function (tree) {

tree.URL = function (val, paths) {
    if (val.data) {
        this.attrs = val;
    } else {
        // Add the base path if the URL is relative and we are in the browser
        if (!/^(?:https?:\/|file:\/)?\//.test(val.value) && paths.length > 0 && typeof(window) !== 'undefined') {
            val.value = paths[0] + (val.value.charAt(0) === '/' ? val.value.slice(1) : val.value);
        }
        this.value = val;
        this.paths = paths;
        this.is = 'uri';
    }
};
tree.URL.prototype = {
    toCSS: function () {
        return this.value.toCSS();
    },
    eval: function (ctx) {
        return this.attrs ? this : new(tree.URL)(this.value.eval(ctx), this.paths);
    }
};

})(require('mess/tree'));

Support local URL arguments. Works on #27 but needs work to support remote datasources