code: func (tc *templateChecker) checkKey(key string) bool {
  	if key == "ij" {
  		return true
  	}
  	for _, param := range tc.params {
  		if param == key {
  			return true
  		}
  	}
  	for _, varName := range tc.letVars {
  		if varName == key {
  			return true
  		}
  	}
  	for _, varName := range tc.forVars {
  		if varName == key {
  			return true
  		}
  	}
  	return false
  }
docs: // checkKey returns true if the given key exists as a param or {let} variable.