var parse = function (x) {
    var temp = [];
    var pushUn = function (a) {
        temp.push(a(temp.pop()));
    };
    function isNumber(n) {
        return !isNaN(parseFloat(n)) && isFinite(n);
    }
    var pushBi = function (a) {
        var r = temp.pop();
        var l = temp.pop();
        temp.push(a(l, r));
    };
    var tokens = x.split(' ');
    for (var i = 0; i < tokens.length; i++) {
        if(tokens[i] in vars){
            temp.push(variable(tokens[i]));
        }
        else if(tokens[i] in binOps){
            pushBi(binOps[tokens[i]]);
        }
        else if(tokens[i] in unOps){
            pushUn(unOps[tokens[i]]);
        }
        else if(isNumber(tokens[i])){
            temp.push(cnst(parseInt(tokens[i])));
        }
    }
    return temp.pop();
};