code: func (v *Vars) lookup(id ID) *varValue {
  	for idx := len(v.scopes) - 1; idx >= 0; idx-- {
  		if val, ok := v.scopes[idx].values[id]; ok {
  			return &val
  		}
  	}
  	return nil
  }
docs: // lookup finds the variable's value in the current scope or any of parent
  // scopes.