What is an unsaturated cyclic compound, C5H4O2, of which two varieties are known, a and g. g-pyrone is the parent substance of several natural yellow dyestuffs called?

pyrone — An unsaturated cyclic compound, C5H4O2, of which two varieties are known, a and g. g-pyrone is the parent substance of several natural yellow dyestuffs. (Webster's 1913.)