HasInterface returns true if err contains an error which implements the interface pointed to by referenceInterface.

HasType returns true iff err contains an error whose concrete type matches that of referenceType.

If iterates on the error's causal chain and returns a predicate's return value the first time the predicate returns true.

Is determines whether one of the causes of the given error or any of its causes is equivalent to some reference error.

IsAny is like Is except that multiple references are compared.

Mark creates an explicit mark for the given error, using the same mark as some reference error.