  Borocarbide compounds with the formula RNi2B2C show interesting
superconducting and magnetic properties and the coexistence of the two
phenomena. BCS theory is extended to systems with underlying commensurate
magnetic order. In the case of helical phases the technique may be extended to
any Q-vector and there exists a well defined limit for incommensurate values.
The way magnetic order influences superconductivity depends crucially on the
details of both the magnetic structure and the electron bands, but some
qualitative criteria may be given. As an example we give a brief analysis of
the compound HoNi2B2C.
