Both Uses

preemptive
in
The Reptile Room
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"It is because this man who is calling himself Stephano," he said, "is really a member of the Herpetological Society, and he is here to try and find the Incredibly Deadly Viper so he can preempt my presentation.†
p. 61.9
Do you three know what the word 'preempt' means?†
p. 61.9 *

Definitions:

(1) (preemptive)
done before something undesired happens to prevent it from happening
(2) (meaning too rare to warrant focus)
The form, preempt can also mean to replace something (typically less important). For example, "The President's speech will preempt our regular broadcast."

Preemptive rights refers to contractual rights that give a party the opportunity to take action (such as buying shares) before others.

In the card game bridge a preemptive bid is an early (usually opening) bid that is so high it interferes with opponents opportunity to exchange information by bidding.