Question: About alias: How can one convert back to an expression?

Hi!

I use 'alias' to define variable dependencies.
But when I want to use the result of my calculation as an expression, I bite the dust; I cannot get rid of the dependencies.
Can anyone help me here?

In the simplified example below, I use the independent variables z[3] and z[4] to define z[1](z[3],z[4]) and z[2](z[3],z[4]).
I then introduce a function f[1](z[3],z[4]) and its derivatives df_dz.
I introduce the names dz[1]/dz[3]=Dp[1,3], dz[1]/dz[4]=Dp[1,4] etc...
When I have calculated df_dz[3], however, I cannot insert values of z[2] without affecting Dp[2,3].
Actually, now I have finished my calculation and want df_dz[2,3] to be a regular expression, so that I can insert values of
z without affecting the Dp:s.
How can I convert df_dz[3] to a regular expression?

alias( seq(z[i]=z[i](seq(z[j],j=3..4)),i=1..2)):
alias(f=f(seq(z[j],j=3..4))):
alias(seq(seq(Dp[m,n]=diff(z[m](seq(z[j],j=3..4)),z[n]),m=1..2),n=3..4)):

f:=z[1]-z[2]*z[3]:

for j from 3 to 4 do
df_dz[j]:=diff(f,z[j]);
od:

df_dz[3];
subs(z[2]=4711,df_dz[3]);
whattype(Dp[1,3]);
whattype(z[3]);




function

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