`\norm{SAx} \approx_\epsilon \norm{Ax}` for all `x`
`\norm{S(Ax-b)} \approx \norm{Ax-b}` for all `x`
then`\min_x \norm{S(Ax-b)}`
is small, within `1+\epsilon` of opt`\quad\quad\norm{A\tilde x - b}`
`\norm{Sy} \approx \norm{y}` for all `y\in C(A)\equiv \{Ax\mid x\in\reals^d\}`