For purposes of this part, the term “issuing or assuming a stock option in a transaction to which section
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(a) applies” means a substitution of a new option for the old option, or an assumption of the old option, by an employer corporation, or a parent or subsidiary of such corporation, by reason of a corporate merger, consolidation, acquisition of property or stock, separation, reorganization, or liquidation, if—
For purposes of this subsection, the parent-subsidiary relationship shall be determined at the time of any such transaction under this subsection.