replaced apply-style proof for instance Multiset :: plus_ac0 by recommended Isar proof style
L2 = Base +
rules
pi_bs "[| A:[]; !!x. x:A ==> B(x):* |] ==> Prod(A,B):*"
lam_bs "[| A:[]; !!x. x:A ==> f(x):B(x); !!x. x:A ==> B(x):* |]
==> Abs(A,f) : Prod(A,B)"
end