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.
The code for one of the branches resulting from a condition.
It can be the branches in a if-else statement or a case branch for languages such as C/C++.
The important aspect is that the branch is not an expression. It can't be evaluated to a value of a type.