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Predicate login_existing_user/1
 login_existing_user(+Claim) is semidet[multifile, library(google_client)]
Called after establishing the identify of the logged in user. Claim is a dict containing
String that uniquely indentifies the user inside Google.
Email address of the user.
Present if oauth_authenticate/3 was called with the option client_data(Term). Note that the term passed is a copy.

This call must return an HTML document indicating that the user logged in successfully or redirect to the URL supplied with return to using http_redirect/3.

 google_client:login_existing_user(+Claim) is semidet[multifile, 'openid.pl']
True if the user is know to us and thus we can perform the login without further interaction.