title: How to add a callable function to a Sympy expression
body: <p>If I have an expression <code>x = Symbol('x')</code> and <code>f1=x**2</code> and I want to further add some <code>f2</code> where <code>f2 = interp1d(t, y)</code> is scipy interpolation. How does one turn <code>f2</code> into an expression such that I have somthing like <code>f = x**2 + f2(x)</code> so that I can later evaluate <code>f</code> as <code>f.subs(x, some_number)</code>?</p>
  <p>Due to specification of the code, I can't evaluate <code>f1</code> and <code>f2</code> separatelly and then add the resulting numbers, I need to be able to add it to an existing sympy expression and evaluate it using something like <code>.subs()</code></p>