title: Use LaTeX symbols in sympy expressions
body: <p>I would like to do some calculations and then have those plotted using sympy. I want to use consistent notation, so I need to define a symbol which will be printed as $\Delta_l^y$.</p>
  
  <p>What I have tried so far:</p>
  
  <pre><code>delta__i_0 = sympy.symbols('Delta__i_0')
  sympy.pprint(delta__i_0)
  </code></pre>
  
  <p>which works fine.
  Unfortunately</p>
  
  <pre><code>delta__y_l = sympy.symbols('Delta__y_l')
  sympy.pprint(delta__y_l)
  </code></pre>
  
  <p>does not really look nice. Any ideas?</p>