What do "$\pm_s$" and "$\mp_t$" mean in a formula (in particular, one for the roots of a quartic polynomial)? I am going through Wikipedia's solution of a quartic equation and have got stuck with notation in the solution. The solution is given by this equation:

$$x=-\frac{B}{4 A}+\frac{\pm_{s} W \mp_{t} \sqrt{-\left(3 \alpha+2 y \pm_{s} \frac{2 \beta}{W}\right)}}{2}$$

What is confusing me is the significance of the following two symbols $\pm_s$ and $\mp_t$.
Can anyone explain what they mean please? I apologise for the relative simplicity of the question.

It says:

Note: The subscript $s$ of $\pm_s$ and $\mp_s$ is to note that they are dependent.

It also says:

Remember: The two $\pm_s$ come from the same place in equation (7'), and
  should both have the same sign, while the sign of $\pm_t$ is independent.

Thus, in other words, these symbols give you a convenient way of writing down all four solutions in a single formula. Those four solutions are characterised by the choice of signs in the formula; there is $\pm_s$ and $\pm_t$, which are independent, and hence together give you 4 choices: $(\pm_s, \pm_t) =(+,+), (+,-), (-,+)$ and $(-,-)$.
