I'm a little flummoxed in trying to move some directories around in 
svr2. Shouldn't the following work?

     mkdir a
     mkdir b
     mv a b

I get the following error:
mv: b exists

I tried many of the possible variants including:

    mv a b/
    mv: b/ exists
    mv a b/a
    mv: directory rename only
    cd b
    mv ../a .
    mv: . exists
    mv ../a ./
    mv: ./ exists
    mv ../a ./a
    mv: directory rename only


If moving directories into existing directories wasn't allowed in those 
days, 1) how were directories managed? and 2) when did moving 
directories into directories become a thing?