error

type Caller interface {
	// File returns the file in which the call occurred.
	File() string

	// Func returns the name of the function in which the call occurred.
	Func() string

	// Line returns the line number in the file in which the call occurred.
	Line() int

	// Pc returns the program counter.
	Pc() uintptr

	// Trace returns the call stack.
	Trace() Trace
}

type Error interface {
	// Caller returns the associated Caller instance.
	Caller() Caller

	// Error implements error.
	Error() string

	// Has tests to see if the test error exists anywhere in the error
	// stack.
	Has(test error) bool

	// Is tests to see if the test error matches most recent error in the
	// stack.
	Is(test error) bool

	// Unwrap returns the next error, if any.
	Unwrap() Error
}

type Trace []Caller