title: Tokenize math expression - error undefined procedure
body: <p>I am trying to tokenize math expression from user's input. I have written this code, but I don't understand, why the predicate "tokenize" isn't defined. I looked up to the definition, but I still don't see the problem.</p>
  
  <pre><code>input_infix:-
      write('Enter equation: '),
      readln(Input),
      tokenize(Input,Chars),
      writeln(Chars).
  
  ERROR: Undefined procedure: tokenize/2
  ERROR: In:
  ERROR:    [9] tokenize(['(',1|...],_1236)
  ERROR:    [8] input_infix at d:/dokumenty/prolog/notation.pl:4
  ERROR:    [7] &lt;user&gt;
  </code></pre>