Genus one -- improvements?

[jvoight]

Fiddling with genus one for the example below. This has degree 28 and surprisingly, when we write phi(x,y) = numer/denom, we usually need an "extra zero" making deg(denom) > 28, but in this case it isn't necessary (we magically have deg(numer) = 27 and deg(denom) = 28). This creates some numerical problems in computing with series and we were checking the wrong thing to see if we needed an extra zero.

sigma := [Sym(28) | (2, 21, 10)(3, 26, 20)(4, 8, 25)(5, 14, 7)(6, 15, 13)(9, 11, 24)(12, 16, 
        23)(17, 28, 22)(18, 19, 27),
    (1, 12, 6, 4, 11, 18, 16)(2, 22, 21, 5, 19, 9, 14)(3, 17, 10, 28, 20, 8, 
        13)(7, 24, 25, 26, 15, 23, 27),
    (1, 16)(2, 17)(3, 28)(4, 24)(5, 27)(6, 8)(7, 9)(10, 22)(11, 19)(12, 15)(13, 
        26)(14, 21)(18, 23)(20, 25)
];
Gamma := TriangleSubgroup(sigma);
SetVerbose("Shimura", true);
X, phi := BelyiMap(Gamma : prec := 100);