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[tex]|\langle a|\psi\rangle|^2[/tex]

where [itex]\langle a|[/itex] is the normalized eigenbra with eigenvalue [itex]a[/itex].

This is more-or-less the formulation of the Born rule as it appears in my text. But this seems to only make sense if [itex]\langle a|[/itex] is non-degenerate. So, what's the rule if we have a degeneracy?