General::altel

Details

This message is generated when an unusual expression based on Alternatives is encountered in a position where an assumption is expected.
This message is most often caused by an incorrect use of Alternatives in an assumption.
Alternatives[p,q,r] can be entered using the notation p|q|r.
Off[message] switches off the message; On[message] switches it on. For example: Off[General::altel].

The second argument in Simplify involves an unusual use of Alternatives:

This shows a typical use of Alternatives in the specification of assumptions:

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