(****
 *
 * Yet one more test of union projection/injection.  I don't remember right now
 * the Tennet def of these terms, but in seems to me that injection means take
 * a union component type and turn it into (i.e., inject it into) the union
 * type, whereas projection means take a union type and "project out" to one of
 * its component types.  By these defs, injection is automatic whereas
 * projection must be done explicitly via the '.' union op (or .< instance
 * extraction op).  It would seem that this scheme of automatic injection but
 * non-automatic projection is consistent with polymorphic value binding, where
 * injection, i.e., binding a sub-class value to a parent-class type, is
 * automatic, whereas projection, i.e., binding a parent-class value to a
 * sub-class type, requires a down-cast (cf. ./PolymorphismCheck.java).
 *
 *)

obj U = i:integer or s:string;
op Op(U, integer)->U;
op main(u:U, i:integer) = (
    Op(u, i);    -- obviously fine
    Op(i, i);    -- OK via union analog of polymorphism rule
    Op(u, u);    -- ERROR, again via union analog of polymorphism rule
    Op(u, u.i);  -- OK via explicit projection (i.e., down-casting)
);