You could solve for `a`

, `b`

, `c`

, `d`

and then ignore `c`

and `d`

.

```
sage: a, b, c, d = var('a b c d')
sage: solns = solve([a+b-1,a-b,c+d],[a,b,c,d]); solns
[[a == (1/2), b == (1/2), c == -r1, d == r1]]
sage: [s[:2] for s in solns]
[[a == (1/2), b == (1/2)]]
```

Or you can tweak `solve`

to only use the equations which involve the variables
that you want to solve for.

```
sage: def smart_solve(eqns, vars):
....: return solve([eqn for eqn in eqns if
....: any(v in eqn.variables() for v in vars)], vars)
....:
sage: smart_solve([a+b-1,a-b,c+d],[a,b])
[[a == (1/2), b == (1/2)]]
```

I don't know what would be the pros and cons of having Sage's `solve`

behave
in this way (either by default or as an option).