Is it possible to obtain fixed value for current %autonumber%?

Question

@startuml
autonumber 1.1

Bob->Alice ++ : Hello!

!$number = "%autonumber%"

note over Bob,Alice : number is $number

return another step

note over Bob,Alice 
I expect the number to remain 1.1 but it is $number

and substr(number,1,1) is "%substr($number,1,1)", not "." as I expect
end note
@enduml
 

Comments

and i tried to write without quotes

!$number = %autonumber%

and got Parsing syntax error about %autonumber%