[isabelle] pushing let environments to the top level

I have a question regarding let environments in formulas. I'd like to push them to the top of the formula and ideally declare them by def-statements in the isar-proof.
Consider e.g. a context with variables x1, ..., xn:
My goal is of the form Psi(s1(X), ... , sm(X)), where X = x1, ..., xn, and
s1(X) of the form

let y1 = t1(X);
    y2 = t2(X,y1);
    yk = tk(X,Y)
in Phi(X,Y,yk)

Now I'd like to move the let-environment to the outside (theoretically this is possible, if no variable capturing occurs, by some beta/eta reduction and expansion):

Psi(s1(X), ... , sm(X))
let y1 = t1(X);
    y2 = t2(X,y1);
    yk = tk(X,Y)
in Psi(Phi(X,Y,yk), s2(X), ... , sm(X))

And then put the let-env to the isar context by def:

def y1 == t1(X)
def y2 == t2(X,y1)
def yk == tk(X,Y)

Psi(Phi(X,Y,yk), s2(X), ... , sm(X))

I'm not a really advanced user so I'd prefer to use an existing procedure to do that automatically. Is there a tactic which does exactly this job? Of course then I won't have the defs visible in my isar proof, and I'd need a labeling convention for this knowledge (to access the equalities yi == t1(...)). The method should also fail if the Y,yk and X intersect in the variable names (variable capturing). I don't think there is already something which does that exactly, but perhaps there is a better way to deal with let environments deep inside formulas
(I don't want to expand/unfold the let environment!) ?

Thanks for your help in advance,


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