title: Prolog procedure does not exist (cant run rules in prolog)
body: <p>I am new to Prolog and using the SWISH SWI online PROLOG website. <a href="https://swish.swi-prolog.org/" rel="nofollow noreferrer">https://swish.swi-prolog.org/</a>
  I am trying a really basic program:</p>
  <pre><code>a('jae').
  b('lii').
  c('jackson').
  happy(A):-sings(A).
  happy(B):-dances(B).
  goToPlay(C):-free(C).
  </code></pre>
  <p>trying to run <em>happy(jae).</em> gives the following error</p>
  <pre><code>procedure `sings(A)' does not exist
  Reachable from:
        happy(A)
  </code></pre>
  <p>Please help me solve this.</p>