  In this work, we establish a few exact identities through commutation of
intra-orbital and inter-orbital on-site pairings with a two-orbital model
describing newly discovered FeAs-based superconductors. Applying the conclusion
drawn from rigorous relation and physical interpretation, we give constraints
on the possible symmetries of the superconducting pairing of the model. Hence
the favorable pairings in newly discovered high-temperature oxypnictide
superconductors are proposed.
