_repr_ description of sum morphisms

[grhkm21]

It seems that the repr of sum and composite morphisms are different. Is that intentional?

sage: set_random_seed(13337)
....: E = EllipticCurve(GF(103), [31, 84])
....: assert E.order() % 7 == 0
....: P = E.random_point() * (E.order() // 7)
....: assert P.order() == 7
....: phi = E.isogeny(P)
sage: phi
Isogeny of degree 7 from Elliptic Curve defined by y^2 = x^3 + 31*x + 84 over Finite Field of size 103 to Elliptic Curve defined by y^2 = x^3 + 42*x + 35 over Finite Field of size 103

sage: phi + phi
Sum morphism:
  From: Elliptic Curve defined by y^2 = x^3 + 31*x + 84 over Finite Field of size 103
  To:   Elliptic Curve defined by y^2 = x^3 + 42*x + 35 over Finite Field of size 103
  Via:  (Isogeny of degree 7 from Elliptic Curve defined by y^2 = x^3 + 31*x + 84 over Finite Field of size 103 to Elliptic Curve defined by y^2 = x^3 + 42*x + 35 over Finite Field of size 103, Isogeny of degree 7 from Elliptic Curve defined by y^2 = x^3 + 31*x + 84 over Finite Field of size 103 to Elliptic Curve defined by y^2 = x^3 + 42*x + 35 over Finite Field of size 103)

sage: E1 = phi.codomain()
....: psi = E1.isogeny(E1.random_point())
....: psi * phi
Composite morphism of degree 49 = 7^2:
  From: Elliptic Curve defined by y^2 = x^3 + 31*x + 84 over Finite Field of size 103
  To:   Elliptic Curve defined by y^2 = x^3 + 11*x + 52 over Finite Field of size 103

I think the "Via: " part should be hidden away. What do you think @yyyyx4

[yyyyx4]

Indeed, unifying and streamlining the repr() and str() of various types of elliptic-curve morphisms (i.e., EllipticCurveHom children) has been an item on my list for a long time. Feel free to take care of it if you like.

[grhkm21]

I'll remove the via part & move all str/repr methods to SchemeMorphism (or higher) in #37705 then.

[grhkm21]

Nevermind, as described in the PR, EllipticCurveHom cannot be merged with SchemeMorphism right now.