KQL addresses an open extension property as extensions.<name>.<property>.
The literal picks the typed sibling: a number or date-time range asks the
number or date sibling, an equality asks every sibling the literal fits
(the lowercased string, and the number, boolean or date it also reads
as), = asks the string as written. Both compilers build the same plan.
The parity suite covers the queries and the lifecycle of extensions
through re-typing, removal, move and trash.
Dominik Schmidt
cafd791fde