for D1, D2 being non empty set
for d1 being Element of D1
for d2 being Element of D2
for p being FinSequence of FinTrees [:D1,D2:] ex p2 being FinSequence of FinTrees D2 st
( dom p2 = dom p & ( for i being Nat st i in dom p holds
ex T being Element of FinTrees [:D1,D2:] st
( T = p . i & p2 . i = T `2 ) ) & ([d1,d2] -tree p) `2 = d2 -tree p2 )