Union functions

Union functions are used to perform unions on values.

Function	Description	Example	Result
union_value(tag := value)	create a union with given value	union_value(a := 1)	1
union_tag(union)	return the tag of union	union_tag(union_value(a := 1))	a
union_extract(union, tag)	return the value for given tag	union_extract(union_value(a := 1), 'a')	1