= Solution
A <forcing> <collapses a cardinal> $\kappa$ over $M$ if it forces that the ground <ordinal> $\kappa$ is no longer a <cardinal>: in the extension there is a <bijection> from some smaller <ordinal> onto $\kappa$. Equivalently there is a <surjection> onto $\kappa$ from an <ordinal> whose ground <cardinality> is smaller. <Forcing> preserves the <ordinal> itself; \b[collapse changes <cardinality>, not the identity of the <ordinal>]. If the assertion is about one particular <generic extension>, the witnessing collapse is interpreted there; a forcing-wide assertion means it is forced by the weakest condition.
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