 All the programs later in the process that make changes to the netlist also, in theory, ensure that these changes are logically equivalent to a previous version.In practice, programs have bugs and it would be a major risk to assume that all steps from RTL through the final tape-out netlist have been performed without error.Also, in real life, it is common for designers to make manual changes to a netlist, commonly known as Engineering Change Orders, or ECOs, thereby introducing a major additional error factor.