Statement being argument does not play with utility methods, where we expect the argument to be present. The vast majority of statements are required to have an argument, but there a few (like input/output) which do not.

When @Nullable annotation comes into play this creates a rather nice conundrum in DeclaredStatement.findFirstDeclaredSubstatementArgument() – we really want to allow only searching for statements which have an argument, otherwise the optional could be interpreted in differing ways.

We either capture the non-null of argument in an intermediate interface, or move the method completely into a sub-interface. This may be sticky as the class hierarchy will get more complicated, hence this should be part of the next release.