# extends_protocol/2-3

## Description

    extends_protocol(Protocol, ParentProtocol)
    extends_protocol(Protocol, ParentProtocol, Scope)

Enumerates, by backtracking, all pairs of protocols such that the first one extends the second. The relation scope is represented by the atoms `public`, `protected`, and `private`.

## Modes and number of proofs

    extends_protocol(?protocol_identifier, ?protocol_identifier) - zero_or_more
    extends_protocol(?protocol_identifier, ?protocol_identifier, ?scope) - zero_or_more

## Errors

`Protocol` is neither a variable nor a valid protocol identifier:
`type_error(protocol_identifier, Protocol)`
`ParentProtocol` is neither a variable nor a valid protocol identifier:
`type_error(protocol_identifier, ParentProtocol)`
`Scope` is neither a variable nor an atom:
`type_error(atom, Scope)`
`Scope` is an atom but an invalid entity scope:
`domain_error(scope, Scope)`

## Examples

    % enumerate the protocols extended by the listp protocol:
    | ?- extends_protocol(listp, Protocol).

    % enumerate protocols that privately extend the termp protocol:
    | ?- extends_protocol(Protocol, termp, private).

See also

current_protocol/1, implements_protocol/2-3, conforms_to_protocol/2-3