title: sympy: substitute function into expression
body: <p>I have a 2D curve rho(t) </p>
  
  <pre><code>import sympy as sy
  
  t = sy.Symbol('t', is_real=True) # curve parameter
  rho_x = sy.Function('rho_x', is_real=True)(t)
  rho_y = sy.Function('rho_y', is_real=True)(t) # components
  rho = sy.Matrix([rho_x, rho_y]) # curve vector
  tau = rho.diff() / rho.diff().norm() # tangent
  </code></pre>
  
  <p>after that I want to evaluate parameters of the curve for a specific realization:</p>
  
  <pre><code>real_shape = {rho_x: sy.sin(t), rho_y: sy.cos(t)}
  f1 = tau.subs(real_shape)
  print f1
  f2 = sy.lambdify(t, f1)
  print f2(0.1)
  </code></pre>
  
  <p>I found that SymPy does not evaluate the derivatives of sine and cosine functions automatically, and after calling <code>print f2(0.1)</code> it shows error message:</p>
  
  <blockquote>
    <p>NameError: global name 'Derivative' is not defined</p>
  </blockquote>
  
  <p>What is wrong in my example?</p>