- This message is generated when a pattern based on Optional appears in an invalid location within another pattern expression.
- A pattern based on Optional cannot be used as the first argument in PatternTest, Condition, Repeated, RepeatedNull, or Optional, or as the second argument in Pattern.
- Optional[p,v] can also be entered using the notation p:v.
- Off[message] switches off the message; On[message] switches it on. For example: Off[General::patop].