Fixtype of statements [C:6.8].
This is a tagged union type, introduced by fty::deftagsum.
We capture almost all the statements:
labeled [C:6.8.1],
compound [C:6.8.2],
expression and null [C:6.8.3],
selection [C:6.8.4],
iteration [C:6.8.5] (with the limitation explained below),
and jump [C:6.8.6].
We do not allow declarations in