Q about UML Negative CombinedFragment

What's the relationship between interactions inside a negative
CombinedFragment and its surrounding interactions? Suppose a negative
CombinedFragment without a guard contains only one method invocation
m() and its corresponding return. And the same invocation m() and its
return follows the negative CombinedFragment. Consider a trace <m(),
return m()>, is this trace valid or invalid?

The UML 2.0 superstructure specification only says "The
interactionOperator neg designates that the CombinedFragment represents
traces that are defined to be invalid." Can someone show me some
examples about its usage? Thanks a lot!

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