MIM2016 - Functions in Inbound Sync Rule

I know that the Function evaluator is a bit.. ehm.. "quirky". 

I would like to concatenate two strings in an inbound Sync Rule.

Currently the Custom Expression looks like this (and it's doing what I expect it to do):

IIF(IsPresent(Cordless),IIF(Eq(Cordless,"0"),Null(),IIF(Eq(Left(Cordless,2),"00"),Mid(Cordless,3,60),Cordless)),Null())

However, I would like to concatenate a string with the result of the Mid Function:

IIF(IsPresent(Cordless),IIF(Eq(Cordless,"0"),Null(),IIF(Eq(Left(Cordless,2),"00"),"46"+Mid(Cordless,3,60),Cordless)),Null())

This is not a valid expression according to MIM2016.

Any suggestions, other than using a Rules Extension or actually changing the inbound data?

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