title: How to process formula by prolog?
body: <p>If</p>
  
  <pre><code>a+b+c=1
  a^2+b^2+c^2=2
  a^3+b^3+c^3=3
  </code></pre>
  
  <p>then</p>
  
  <pre><code>a^4+b^4+c^4=?
  </code></pre>
  
  <p>I've known the result is 25/6,but how to calculate it by prolog?</p>
  
  <p>I tried this but failed:</p>
  
  <pre><code>[1] 5 ?- A+B+C=:=1,A**2+B**2+C**2=:=2,A**3+B**3+C**3=:=3.
  ERROR: Unhandled exception: =:=/2: Arguments are not sufficiently instantiated
  </code></pre>