code: func (g *Graph) evalBool(e *Graph) bool {
  	i, err := g.Eval(e)
  	if err != nil {
  		return false
  	}
  	b, _ := _boolf(i)
  	return b
  }
docs: // EvalBool takes a parsed expression and evaluates it in the context of the
  // current graph, and converts the result to a boolean.