title: Labeling in Prolog constraint programming
body: <p>I am new to Prolog and currently working on a simple constraint programming problem. So I have four real numbers A,B,C,D with the property such that
  A+B+C+d = A<em>B</em>C*D = 7.11
  Since it is easier to work with integer, I tried the following implementation:</p>
  
  <pre><code>   :- use_module(library(clpfd)).
     grocery(Vars):-
        Vars=[A,B,C,D],
        X #= 100 * A,
        Y #= 100 * B,
        Z #= 100 * C,
        W #= 100 * D,
        X+Y+Z+W #= 711,
        X*Y*Z*W #= 71100000000.
  </code></pre>
  
  <p>Since the above will give me partially solved answer, I tried putting the keyword <code>label(Vars)</code> at the end. But this causes my execution of <code>grocery(V)</code> to produce</p>
  
  <pre><code>ERROR: Arguments are not sufficiently instantiated.
  </code></pre>
  
  <p>While <code>grocery([V])</code> will give me a <code>false</code>. Can anyone enlighten me on how to do the labeling? Thanks</p>
  
  <p>Edit : I did not put the call to the library clpfd earlier on</p>