def decodePolymorphicNucleotide(nuc) :
	"""the opposite of encodePolymorphicNucleotide, from 'R' to ['A', 'G']"""
	if nuc in polymorphicNucleotides :
		return polymorphicNucleotides[nuc]

	if nuc in nucleotides :
		return nuc

	raise ValueError('nuc: %s, is not a valid nucleotide' % nuc)