Support subs

[softaria-roman]

1) tests sympy client difF[der[lvar[t],84],const[pi],2]:
     AssertionError: Unsupported constructionSubs/Unknown function: 

Subs(Derivative(Symbol('t'),Tuple(Symbol('t'),Integer(84))),Tuple(Symbol('t')),Tuple(Integer(2)))/Add(Subs(Derivative(Symbol('t'),Tuple(Symbol('t'),Integer(84))),Tuple(Symbol('t')),Tuple(Integer(2))),Mul(Integer(-1),Subs(Derivative(Symbol('t'),Tuple(Symbol('t'),Integer(84))),Tuple(Symbol('t')),Tuple(pi))))

      at Context.<anonymous> (tsOutputs/test/sympy-client-bdd.js:71:40)
      at Generator.throw (<anonymous>)
      at rejected (tsOutputs/test/sympy-client-bdd.js:5:65)
      at processTicksAndRejections (internal/process/task_queues.js:93:5)