dictionary substitution for elements of ExteriorAlgebra

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As a newbie for sage, I don't understand how to perform substitution for elements in, e.g., an ExteriorAlgebra.

For example, i'm trying:

E.<x,y>=ExteriorAlgebra(SR)
x.substitute({x:y})

and i'd expect "y", instead getting "x". It seems substitution in this case is not possible... So, where am i getting wrong, and what is the correct way to perform such an operation?

thanks