Allow generation of EMPLACE_TYPE

for variant if all oplists defined a FIELD operator.

README.md

@@ -624,6 +624,7 @@ The following classic operators are usually expected for an object:
 Other documented operators are:
 
 * `NAME()` --> `prefix`: Return the base name `prefix` used to construct the container.
+* `FIELD()` --> `field name`: Return the name of the field used when constructing the container.
 * `TYPE()` --> `type`: Return the base type associated to this oplist.
 * `SUBTYPE()` --> `type`: Return the type of the element stored in the container (used to iterate over the container).
 * `GENTYPE()` --> `type`: Return the type representing TYPE suitable for a _Generic statement.