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@@ -183,6 +183,7 @@ The value of the expression given by @code{desc} i
each @code{tag} value, each of which should be different. If no
@code{tag} is matched, the field marked with @code{default} is used if
there is one, otherwise no field in the union will be marked.
+A union field can have at most one @code{tag}.
In the @code{desc} option, the ``current structure'' is the union that
it discriminates. Use @code{%1} to mean the structure containing it.