sub for a sub

[Nevadaref]

This is legal because there is NOT a rule which prohibits it.

The only way that A3 could be denied entry in this situation is if the coach took took long to make his decision and the ball was about to be put into play. In that case, the rule says that the beckoning signal should be withheld. (3-3-2)

There has to be a cutoff point so that the substitution process doesn't take forever.