# Functions
Is returns true if s is a valid sentinel.
# Constants
End is the Unicode special non-character U+FFFF, and the suffix included in all Boundary defined sentinel values.
ExternalIdNone is a Boundary sentinel indicating that no id was provided by an external system.
ErrorDetailsNone is a Boundary sentinel indicating that there were no error details.
Start is the Unicode special non-character U+FFFE, and the prefix included in all Boundary defined sentinel values.