in:foo,bar,...

• The field under validation must be included in the given list of values.

not_in:foo,bar,...

• The field under validation must not be included in the given list of values.

  data = %{
    country: "italy",
    grade: "a",
  }
  rules = [
    %{
      field: "country",
      validate: ["in:iran,italy,usa"]
    },
    %{
      field: "grade",
      validate: ["not_in:c,d,e", "in:a,b"]
    }
  ]