# Polynomial Substitution for only higher terms

I'm dealing with elliptic curves in SAGE. Say the elliptic curve I have is $y^2=x^3+x$ working over GF(43) and I have a polynomial expression in x and y: $$y^2 + xy + 1.$$ I want to be able to substitute $y^2=x^3+x$ to get $(x^3+x)+xy+1$. How do I do this?

n is missing in polyomial