r1: x + 3 = 5;
solve(r1);

r1: x + 3 = 5;
r2: x - 3 = 1;
solve([r1, r2], [x]);

r1: x + y = 5;
r2: x - y = 1;
solve([r1, r2], [x, y]);

r1: y = 5;
solve([r1], [x, y]);

r1: x^2 + 1 = 0;
solve([r1], [x]);