isSolution cells = (not (hasDuplicates (map toColumn cells))) && (not (hasDuplicates (map toRow cells))) && (not diag cells)
    where toColumn (_,c) = c
          toRow(r, _) = r
          toDiag(x,y) = x-y
isSolution cells = (not (hasDuplicates (map toColumn cells))) && (not (hasDuplicates (map toRow cells))) && (not diag cells)
    where toColumn (_,c) = c
          toRow(r, _) = r
          toDiag(x,y) = x-y
isSolution cells = (not (hasDuplicates (map toColumn cells))) && (not (hasDuplicates (map toRow cells))) && (not diag cells)
    where toColumn (_,c) = c
          toRow(r, _) = r
          toDiag(x,y) = x-y