you can check my opensource project that seems to make the same as smath it is called Quantity System http://QuantitySystem.org http://QuantitySystem.org and

you can check my opensource project that seems to make the same as smath it is called Quantity System http://QuantitySystem.org http://QuantitySystem.org and it blends numerical and symbolic in the same context and it supports units in the context of quantities check the specs and tell me your opinion.