code: func (e EventBuilder) Alternatives(alts string) EventBuilder {
  	e.assertPattern()
  	e.fields = append(e.fields, Field{
  		valid:   true,
  		value:   "[" + alts + "]",
  		pattern: patternAlts,
  	})
  	return e
  }
docs: // Alternatives creates a field with the alts wrapped in brackets, to match
  // one of them in a Redis pattern, and chains it on to the event.