def mafDataOrNone( mafBlocksByChrom, c ):
   """ return d which is either the list of MafBlocks
   for a given chromosome, c, or return None.
   """
   if c not in mafBlocksByChrom:
      return None
   if mafBlocksByChrom[ c ] is None:
      return None
   if len( mafBlocksByChrom[ c ] ) < 1:
      return None
   return mafBlocksByChrom[ c ]