DOCUMENTATION:
  name: mandatory
  version_added: "historical"
  short_description: make a variable's existance mandatory
  description:
    - Depending on context undefined variables can be ignored or skipped, this ensures they force an error.
  positional: _input
  options:
    _input:
      description: Mandatory expression.
      type: raw
      required: true
EXAMPLES: |

    # results in a Filter Error
    {{ notdefined | mandatory }}

RETURN:
  _value:
    description: The input if defined, otherwise an error.
    type: raw