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union                                                            (Lib)
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union : ''a list -> ''a list -> ''a list

SYNOPSIS
Computes the union of two ‘sets’.

KEYWORDS
list, set, eqtype.

DESCRIBE
If {l1} and {l2} are both ‘sets’ (lists with no repeated members),
{union l1 l2} returns the set union of {l1} and {l2}. In the case that
{l1} or {l2} is not a set, all the user can depend on is that {union l1 l2}
returns a list {l3} such that every unique element of {l1} and {l2} is in
{l3} and each element of {l3} is found in either {l1} or {l2}.

FAILURE
Never fails.

EXAMPLE

- union [1,2,3] [1,5,4,3];
val it = [2,1,5,4,3] : int list

- union [1,1,1] [1,2,3,2];
val it = [1,2,3,2] : int list

- union [1,2,3,2] [1,1,1] ;
val it = [3,2,1,1,1] : int list




COMMENTS
Do not make the assumption that the order of items in the list
returned by {union} is fixed. Later implementations may use
different algorithms, and return a different concrete result
while still meeting the specification.

A high-performance implementation of finite sets may be found in
structure {HOLset}.

ML equality types are used in the implementation of {union} and its kin.
This limits its applicability to types that allow equality. For other
types, typically abstract ones, use the ‘op_’ variants.

SEEALSO
Lib.op_union, Lib.U, Lib.mk_set, Lib.mem, Lib.insert, Lib.set_eq,
Lib.intersect, Lib.set_diff, Lib.subtract.

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