Concatenation( list1, list2.. )
Concatenation( list )
In the first form Concatenation returns the concatenation of the lists
list1, list2, etc. The concatenation is the list that begins with
the elements of list1, followed by the elements of list2 and so on.
Each list may also contain holes, in which case the concatenation also
contains holes at the corresponding positions.
gap> Concatenation( [ 1, 2, 3 ], [ 4, 5 ] );
[ 1, 2, 3, 4, 5 ]
gap> Concatenation( [2,3,,5,,7], [11,,13,,,,17,,19] );
[ 2, 3,, 5,, 7, 11,, 13,,,, 17,, 19 ]
In the second form list must be a list of lists list1, list2, etc,
and Concatenation returns the concatenation of those lists.
gap> Concatenation( [ [1,2,3], [2,3,4], [3,4,5] ] );
[ 1, 2, 3, 2, 3, 4, 3, 4, 5 ]
The result is a new list, that is not identical to any other list. The elements of that list however are identical to the corresponding elements of the argument lists (see Identical Lists).
Note that Concatenation creates a new list and leaves it arguments
unchanged, while Append (see Append) changes its first argument. As
usual the name of the function that works destructively is a verb, but
the name of the function that creates a new object is a substantive.
Set(Concatenation(set1,set2..)) (see Set) is a way to compute the
union of sets, however, Union (see Union) is more efficient.
GAP 3.4.4