R+ cell setting:

  A=B=C, alpha=beta=gama


Hexagonal cell:

  a=b, c, 90, 90 ,120


Cell parameter converstions:

  a^2 = (A^2)/3 + (C^2)/9

  cos(alpha)= (2 C^2 - 3 A^2) / (2 C^2 + 6 A^2)


Reindex from an R-cell to a hexagonal cell:

  H=h-k

  K=  k-l

  L=h+k+l


Conditions limiting possible reflections:

  -H+K+L=3k