text: LeftAttribute = function(obj) {
    this.perceive(obj);
  }
  LeftAttribute.prototype.key = "left_pos";
  LeftAttribute.prototype.targetType = 'obj';
  LeftAttribute.prototype.arity = 1;
  LeftAttribute.prototype.size = 100; // scene size
  LeftAttribute.prototype.constant = false;
  
  LeftAttribute.prototype.membership = function(x) {
  	return 1-1/(1+Math.exp(20*(0.4-x/this.size)));
  }
  
  LeftAttribute.prototype.perceive = function(obj) {
    this.obj = obj;
    this.val = obj.x;
  }
  
  LeftAttribute.prototype.get_activity = function() {
    return this.membership(this.val);
  }
  
  LeftAttribute.prototype.get_label = function() {
    return 'left';
  }
  