$AL
is the head for dummy indices which may be introduced by Amputate
and Uncontract
. By default it is unset, but may be set to anything.
Overview, Amputate, Uncontract.
[ScalarProduct[p, q], q, Pair -> All] Uncontract
= \[Mu];
$AL
[ScalarProduct[p, q], q, Pair -> All] Uncontract
=.; $AL