Victor,
%Object create o ::o %o attribute {a oldvalue} ::o::a %o a oldvalue %o attribute {a newvalue} ::o::a %o a oldvalue
Forgot to ask: Why do you take this path (changing the default of the attribute slot) in the first place? If you want "a" to have the value "newvalue" assigned, why don't you simply say:
o a newvalue
//s