title: Define a variable in sympy to be a CONSTANT
body: <pre><code>from sympy import *
  from sympy.stats import *
  mu, Y = symbols('mu Y', real = True, constant = True)
  sigma = symbols('sigma', real = True, positive=True)
  X = Normal('X', mu, sigma)
  </code></pre>
  
  <p>When asking for:</p>
  
  <pre><code>E(X, evaluate=False)
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>∞                     
  ⌠                     
  ⎮                2    
  ⎮        -(X - μ)     
  ⎮        ──────────   
  ⎮              2      
  ⎮           2⋅σ       
  ⎮  √2⋅X⋅ℯ             
  ⎮  ──────────────── dX
  ⎮       2⋅√π⋅σ        
  ⌡                     
  -∞ 
  </code></pre>
  
  <p>Which is what I expect. When asking for:</p>
  
  <pre><code>E(X, X&gt;0, evaluate=False)
  E(X, X&gt;pi, evaluate=False)
  E(X, X &gt;-3, evaluate=False)
  </code></pre>
  
  <p>Using any constant, the result is as expected from the Normal Definition of conditional expectation. However, when trying to solve for:</p>
  
  <pre><code>E(X, X&gt;Y)
  </code></pre>
  
  <p>I'm getting an error that has to do with roots. Is there a way to define a Y, such that sympy acknowledges that it is a constant, just like a 0 or a -3 or even pi, and shows the integration as expected? I'm assuming the problem with the request I have from sympy is that somehow the Y isn't acknowledges as a constant and therefore, when trying to solve this request, sympy is faced with a roots problem.</p>