code: func (fb *Firebase) EqualToValue(value interface{}) *Firebase {
  	c := fb.copy()
  	// explicitly not locking here because no one else can
  	// modify this value before we return it.
  	if value != "" {
  		c.params.Set(equalToParam, escapeParameter(value))
  	} else {
  		c.params.Del(equalToParam)
  	}
  	return c
  }
docs: // EqualToValue sends the query string equalTo so that one can find nodes with
  // exactly matching values. The value that is passed in is automatically escaped
  // if it is a string value.
  // Numeric strings are preserved as strings.
  //
  //    EqualToValue(7)        // -> equalTo=7
  //    EqualToValue("7")      // -> equalTo="7"
  //    EqualToValue("foo")    // -> equalTo="foo"
  //    EqualToValue(`"foo"`)  // -> equalTo="foo"
  //
  // Reference https://firebase.google.com/docs/database/rest/retrieve-data#section-rest-filtering