First example in readme uses undefined variable

[mykelk]

I think we just need to define something for n in:

@satvariable(x[1:n], Bool)
@satvariable(y[1:n], Bool)
expr = (x .∧ y) .∨ (¬x .∧ y)
status = sat!(expr, solver=Z3())
println("x = $(value(x)), y = $(value(y))”)