title: Working with statistical expressions in SymPy without specifying distribution
body: <p>Is it possible to manipulate statistical expressions in <code>SymPy</code> without specifying the distributions of random variables?</p>
  
  <p>For example, I would like to work with two random variables, <code>X</code> and <code>Y</code>, and be able to ask <code>SymPy</code> to expand the expression <code>Var(X + Y)</code>, getting the answer <code>Var(X) + Var(Y) + 2 Cov(X,Y)</code>. </p>
  
  <p>I would also like to be able to put restrictions on the distributions of <code>X</code> and <code>Y</code>, for example telling SymPy that <code>Cov(X, Y) = 0</code>.</p>
  
  <p>Is this at all possible in <code>SymPy</code>? If not, is there another package that can do it?</p>