code: func mergeVars(old, new Vars) Vars {
  	vars := make(Vars)
  
  	for n, v := range old {
  		vars[n] = v
  	}
  
  	for n, v := range new {
  		if v == nil {
  			delete(vars, n)
  		} else {
  			vars[n] = v
  		}
  	}
  
  	return vars
  }
docs: // mergeVars copies all of the vars from a, and merges b into them, returning a
  // new Vars.