code: func (ex extensions) Check(ext pkix.Extension, d *certdata.Data) *errors.Errors {
  	var e = errors.New(nil)
  	var found bool
  
  	for _, ec := range ex {
  		if ec.oid.Equal(ext.Id) {
  			found = true
  			if ec.filter != nil && ec.filter.Check(d) {
  				continue
  			}
  			e.Append(ec.f(ext, d))
  		}
  	}
  
  	if !found {
  		// Don't report private enterprise extensions as unknown, registered private
  		// extensions have still been checked above.
  		if !strings.HasPrefix(ext.Id.String(), "1.3.6.1.4.1.") {
  			e.Warning("Certificate contains unknown extension (%s)", ext.Id.String())
  		}
  	}
  
  	return e
  }
docs: // Check lookups the registered extension checks and runs all checks with the
  // same Object Identifier.