Defines what to do with redefinitions during Advice de/activation.
Redefinition occurs if a previously activated function that already has an
original definition associated with it gets redefined and then de/activated.
In such a case we can either accept the current definition as the new
original definition, discard the current definition and replace it with the
old original, or keep it and raise an error. The values `accept', `discard',
`error' or `warn' govern what will be done. `warn' is just like `accept' but
it additionally prints a warning message. All other values will be interpreted as `error'.