title: How to allow for an arbitrary constant in Sage
body: <p>I'm a calculus student, and I frequently use Sage to check work. Often, it's necessary to allow for a constant difference between two expressions. For instance, when finding an antiderivative, the two results <code>ln(2*x)</code> and <code>ln(x)</code> are equivalent, but of course <code>bool(ln(2*x) == ln(x))</code> returns <code>False</code>. Is there a way to define a variable <code>C</code> so that <code>bool(ln(2*x) == ln(x) + C)</code> returns <code>True</code>?</p>