null derefs (1)
introduce a null object
Null : Object!
Null.f = Null (initially for all field relations f)
E ? null ? = Null
make field relations total
define derefs performed in a term
Derefs ? t.f ? = Derefs ? t ?
Derefs ? v.f ? = v
instrument program to collect derefs
derefs : Object
S ? t.f = u ? = … ? derefs’ = derefs + Derefs ? t.f ? + Derefs ? u ?