code: function(s, p, o)
  {
     if(p in s)
     {
        if(s[p].constructor === Array)
        {
           s[p].push(o);
        }
        else
        {
           s[p] = [s[p], o];
        }
     }
     else
     {
        s[p] = o;
     }
  }
docs: Sets a subject's property to the given object value. If a value already
  exists, it will be appended to an array.
  
  @param s the subject.
  @param p the property.
  @param o the object.