whole_func_string: function signout ({username, password}) {
    return fetch(`${YGGDRASIL_API}/signout`, {
      method: 'POST',
      body: JSON.stringify({
        username,
        password
      }),
      headers: {
        'user-agent': USER_AGENT,
        'content-type': 'application/json',
        'accept': 'application/json'
      }
    })
      .then(handleErrors)
      .then(res => null)
  }
func_code_string: function signout ({username, password}) {
    return fetch(`${YGGDRASIL_API}/signout`, {
      method: 'POST',
      body: JSON.stringify({
        username,
        password
      }),
      headers: {
        'user-agent': USER_AGENT,
        'content-type': 'application/json',
        'accept': 'application/json'
      }
    })
      .then(handleErrors)
      .then(res => null)
  }
func_documentation_string: Invalidates access tokens of a given Mojang account.
  
  @param {Object} credentials - account credentials
  @param {string} credentials.username - email or username of a Mojang account
  @param {string} credentials.password - password for the given account
  @returns {Promise} resolves only if credentials were correct
  @see {@link http://wiki.vg/Authentication#Signout}
  @example
  mojang.signout({username, password})
  .then(() => console.log('user signed out'))
  .catch(err => console.error(err))