If the node is blacklisted from being mutated. This is for example when the node covers a C macro.
If the node should not be part of mutant schemata because it is highly likely to introduce compilation errors. It is for example likely when operators are overloaded.
A block of code such such as a local scope enclosed by brackets, {}.
It is intended to be possible to delete it. But it may need to be further analyzed for e.g. Return nodes.