title: Reformulate a fractional expression in sympy using a ratio of symbols
body: <pre><code>from sympy import *
  a,b,c = symbols("a b c")
  f = (a*b + c) / (a*b)
  </code></pre>
  
  <p>How to express <code>f</code> as a function of the ratio "b/c"? </p>
  
  <p>This is what I want to achieve</p>
  
  <pre><code>f = (a*(b/c) + 1) / (a*(b/c))
  </code></pre>
  
  <p>I tried <code>factor(f, 1/c)</code>, <code>factor(f, c)</code> in the attempt to divide with <code>c</code> later, dividing by <code>c</code>, but it doesn't work.</p>