thread id specification

[Rama]

Can anyone in-the-know shed some light on how notmuch generates its
thread ids? Until recently, I'd only seen numeric thread ids 16
characters long, padded with zeroes. For example:

thread:0000000000000001
thread:0000000000000002
thread:0000000000000005
etc etc

Today, several new threads were created with non numeric ids:

thread:000000000000000d
thread:000000000000000e
thread:000000000000000f

Is this normal behavior?

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Rama