SolveSystem​(System.​Collections.​Generic.​IEnumerable{AngouriMath.​Entity},​System.​ReadOnlySpan{AngouriMath.​Entity.​Variable})

Solves a system of equations by solving one after another with substitution, e.g.
let { x - y + a = 0, y + 2a = 0 } be a system of equations for variables { x, y }
Then we first find y from the first equation,
y = x + a
then we substitute it to all others
x + a + 2a = 0
then we find x
x = -3a
Then we substitute back
y = -3a + a = -2a