title: Prolog syntax error: illegal start of term
body: <p>So I'm trying to test my the predicate <code>reduce/4</code> defined as following:</p>
  
  <pre><code>:- op(20,xfy,?=).
  
  reduce(R,X ?=Y,P,Q) :-
      R=rename,
      regle(X ?=Y,R),
      length(P,N),
      trouve(Y,P,N,M),
      remove(N,M,P,Q).
  </code></pre>
  
  <p>but i keep getting this error:</p>
  
  <pre><code>?- reduce(rename,X ?= Y,[?=(X,Y),?=(f(a),g(Z)),?=(X,a)],Q].
  
  ERROR: Syntax error: Illegal start of term
  ERROR: reduce(rename,X ?= Y,[?=(X,Y),?=(f(a),g(Z)),?=(X,a)],
  ERROR: ** here **
  ERROR: Q] . 
  </code></pre>
  
  <p>I'm new to Prolog so maybe it is something stupid.</p>