Fixtype of statements [C17:6.8].
This is a tagged union type, introduced by fty::deftagsum.
We capture almost all the statements:
labeled [C17:6.8.1],
compound [C17:6.8.2],
expression and null [C17:6.8.3],
selection [C17:6.8.4],
iteration [C17:6.8.5]
(except for