title: sympy: PolynomialError: cos(a) contains an element of the generators set
body: <p>While using <code>sympy</code> (current version) to solve an polynomial equation (polynom would be d² in this case):</p>
  
  <pre><code>from sympy import solve_poly_system
  solve_poly_system(4*d**2*sin(a)**2*sin(b)/cos(b)**2 - d*cos(a) + 4, d**2*sin(a)**2*sin(b)/cos(b)**2 - d*cos(a) + 8, 3*d**2*sin(a)**2*sin(b)/cos(b)**2 - d*cos(a) + 3 ,d ,a, b)
  </code></pre>
  
  <p>I receive the following error:</p>
  
  <blockquote>
    <p>PolynomialError: cos(a) contains an element of the generators set</p>
  </blockquote>
  
  <p>What is the exact meaning of this error message. And why does it specifically points to the expression cos(a)?</p>