return header
return (id __nonnull)header;

^(id  _Nullable result, NSError * _Nullable error)
^(SPTMessageTrack *track, NSError *error)

^(id data, NSError *error)
^(SPTMessageAccessToken *token, NSError *error)

fetchAccessTokenWithCallback:(void(^)(id data, NSError *error))block
fetchAccessTokenWithCallback:(void(^)(SPTMessageAccessToken *, NSError *))block

NS_ASSUME_NONNULL_BEGIN
NS_ASSUME_NONNULL_END
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