An easier way to compute 2-cocycles coming from a reduction for semidirect products

For Hamiltonian actions of semidirect products $G=F \ltimes H$, we study 2-cocycles arising from residual Hamiltonian actions of $F$ on Hamiltonian reductions for $H$. The motivation comes from the study of Teichmuller spaces for surfaces with boundary, which carry Hamiltonian actions of the Virasoro algebra. In this paper, we give a general setup for the problem, and we suggest an easier way to obtain the Gelfand-Fuchs 2-cocycles for Hamiltonian actions on Teichmuller spaces.