Two days after it widened the service tax net, the finance ministry is now toying with the idea of having a negative list of services that would not attract the 10% tax."We could have a negative list (of services) at some point of time, but it is the prerogative of the finance minister," Parthasarathy Shome, adviser to finance minister, said at a post-budget meeting organised by CII today.Shome said the objective of tax rationalisation was to provide a level-playing field for service providers."We have expanded the coverage, but kept tax rates intact," Shome said, adding the threshold was increased to clean up the tax system.Both Shome and revenue secretary K M chandrasekhar also explained the rationale for large taxpayer units as promised in the budget.They said the units would provide a single window system for large taxpayers to pay up income tax, service tax, customs and excise duties, and would be beneficial for India Inc since it would do away with many hurdles."Large taxpayers units exist in about 50 countries," Shome said.