CSS code

In quantum error correction, CSS codes, named after their inventors, Robert Calderbank, Peter Shor and Andrew Steane, are a special type of stabilizer codes constructed from classical codes with some special properties. An example of a CSS code is the Steane code.
Construction.
Let formula_1 and formula_2 be two (classical) formula_3, formula_4 codes such, that formula_5 and formula_6 both have minimal distance formula_7, where formula_8 is the code dual to formula_9. Then define formula_10, the CSS code of formula_11 over formula_9 as an formula_13 code, with formula_14 as follows: 
Define for formula_15 formula_16 formula_17, where formula_18 is bitwise addition modulo 2. Then formula_19 is defined as formula_20.