title: sympy index range of vector of symbols
body: <p>Hi I am trying to index a range of a vector of symbols.</p>
  <p>My first idea was to use a MatrixSymbol</p>
  <pre><code>A = MatrixSymbol('A',10,1)
  B = Matrix(A[0:2]) # does not work!
  </code></pre>
  <p>Then I thought I could use an IndexBased, but how do I generate the submatrix B (I can't index a range)</p>
  <pre><code>A=IndexedBased('A')
  B=Matrix([A[0],A[1]]) # works but I want to define a range
  </code></pre>
  <p>There must be a proper way to do this.</p>