code: function GetSubEnvironment (subEnvironmentName) {
    let e = GetSubEnvironments()[subEnvironmentName]
    if (!e) {
      throw new Error('SubEnvironment does not exist.')
    }
    if (!(e instanceof ent.Environment)) {
      throw new Error('SubEnvironment is invalid.')
    }
    return e
  }
docs: Gets a particular sub-Environments of the context, raises error if it's not of type Environment.
  @returns Environment object.
  @public