A tagged union can now optionally use multiple fields to indicate the
tag. These are called "sliced" tags in the code. The tag fields have to
be at the same position and width in each block of the tagged unions,
and all tag fields have to be within the same word.
When sliced tags are used, tag class masks cannot be used at the same
time for this tagged union.
In the `tagged_union` declaration, the values of such sliced tags
become tuples. For instance, if `x` and `y` are tag fields (declared in
blocks not shown here) with width 1 and 2, we can write:
tagged_union ex exType(x,y) {
tag ex1 (0,2)
tag ex2 (1,3)
}
Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>
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