x_add_with_inv := function(a) # this function returns a map that adds 'a' local phi, psi; phi := function(b) return b+a; end; psi := function(c) return c-a; end; return Map(Z,Z,phi,psi); end; x_f := x_add_with_inv(5); x_f(2); x_Z7 := Quotient(Z,7); x_g := Composition(x_Z7.ext1,x_f); x_g(1); Preimage(Coerce(x_Z7,8),x_g);