This works:

plot(tickmarks = [spacing(1), spacing(2)]);

but not this:

plot(axis = [tickmarks = [spacing(1), spacing(2)]]);
Error, (in plot) incorrect value for option tickmarks or gridlines: [spacing(1), spacing(2)]

Seems wrong, and the problem is that for gridlines neither form works, so gridlines=[spacing(1), spacing(2)] doesn't seem to be possible.

int(sqrt(t^4+1)-t^2, t = 1 .. infinity);
       -(1/3*I)*sqrt(2)*(-(2*I)*EllipticK(I)-EllipticK(I*sqrt(-2)))+(1/3*I)*sqrt(2)*EllipticF((1/2)*sqrt(2)+
       (1/2*I)*sqrt(2), I)+(1/3)*sqrt(2)*(-(2*I)*EllipticK(I)-EllipticK(I*sqrt(-2)))-(1/3)*sqrt(2)*
       EllipticF((1/2)*sqrt(2)+(1/2*I)*sqrt(2), I)-(1/3)*sqrt(2)+1/3

The result is the correct value for int(sqrt(t^4+1)-t^2, t = 1 .. -infinity).

Why don't these two work?

(f@seq)(i, i = 1 .. 2);
                        f(i, i = 1 .. 2)

(f@add)(i, i = 1 .. 2);
Error, (in simpl/relopsum) invalid terms in product: 1 .. 2

I was expecting f(1, 2) and f(3).

collect((sqrt(x)+y)^2, [x, y], distributed);
                                       (sqrt(x)+y)^2

I think this is wrong. Documentation says that collect supports fractional powers of the variables, so in this case collect should return x+2*sqrt(x)*y+y^2.

f := (s) -> -HankelH1(1, s)-2*I/Pi*BesselK(1, s);

evalf(Int(f(s), s = 0 .. 1));
                 -0.2348023134 + 0.3562894462 I

evalf(Int(Re(f(1.*s)), s = 0 .. 1)+I*Int(Im(f(1.*s)), s = 0 .. 1));
                 -0.2348023134 + 0.3562894462 I

evalf(Int(f(1.*s), s = 0 .. 1));
               -0.2094198744 + Float(infinity) I

I don't know if the third one is really an issue, but given that the first two work fine, looks suspicious.