def Required(field, dictionary):
    """
    When added to a list of validations
    for a dictionary key indicates that
    the key must be present. This
    should not be called, just inserted
    into the list of validations.
    # Example:
        validations = {
            "field": [Required, Equals(2)]
        }
    By default, keys are considered
    optional and their validations
    will just be ignored if the field
    is not present in the dictionary
    in question.
    """
    return field in dictionary
