Supplying ARRAY/INITIAL in Rebol with a function value initiates each element of the array with the product of a fresh invocation of the function. This doesn't happen in Rebmu. Rebol 3:
>> n: 1 array/initial 4 does [++ n]
== [1 2 3 4]
And in Rebmu:
>> rebmu/debug [I&[a+Jn]Aai~4 i]
Executing: [i: & [a+ j n] a: ai~ 4 i]
== [1 1 1 1]