/* Runtime evaluation in an environment, in Jsish */
function evalWithX(expr, a, b) {
    var x = a;
    var atA = eval(expr);
    x = b;
    var atB = eval(expr);
    return atB - atA;
}

;evalWithX('Math.exp(x)', 0, 1);
;evalWithX('Math.exp(x)', 1, 0);

/*
=!EXPECTSTART!=
evalWithX('Math.exp(x)', 0, 1) ==> 1.71828182845905
evalWithX('Math.exp(x)', 1, 0) ==> -1.71828182845905
=!EXPECTEND!=
*/