substitution of variable in the result of monomials()

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Given a polynomial in n variables, I'd like to extract the list of its monomials, and then manipulate that list by substituting certain variables for others. Simple example: in $Z[x,y]$ consider the polynomial $1+x+y^2$; the list of its monomials (in some ordering) is $(1,x,y^2)$. My function should be able e.g. to take that list and substitute $y$ with $x$, namely return $[1,x,x^2]$.

At the moment, my code gives error, but I do not understand how to fix it.

N.<x1,x2> = PolynomialRing(ZZ, 2)
f = 1+x1+x2
g = f.monomials()
for i in range(3):
   g[i] = g[i].substitute_expression(x2==x1)

Namely, how do I make a variable in the polynomial ring also have the substitute attribute? or is there a better way to achieve this?