http_ax:http_ax_attributes/2

http_ax_attributes(+Spec, -HTTPAttributes) is det

True when HTTPAttributes is a list of Name=Value pairs that can be used with an HTTP request to query for the attributes Spec. Spec is a list of elements Alias(Value[, Options]). Options include:

required: The attribute is required. This is mutually exclusive with if_available.
if_available: Only provide the attribute if it is available. This is mutually exclusive with required. This is the default.
url(+URL): Can be used to ovcerrule or extend the ax_alias/2.
count(+Count): Maximum number of values to provide

For example:

?- http_ax_attributes([ nickname(Nick),
                        email(Email, [required])
                      ], Params).
Params = [ 'openid.ax.mode'          = fetch_request,
           'openid.ax.type.nickname' = 'http://axschema.org/namePerson/friendly',
           'openid.ax.type.email'    = 'http://axschema.org/contact/email',
           'openid.ax.required'      = email,
           'openid.ax.if_available'  = nickname
         ].