plus and minus Operators

For the details on plus and minus operators for maps, see Map Specific Operations. The plusAssign () and (-=) are also defined for collections. However, for read-only collections, they actually use the plus or minus operators and try to assign the result to the same variable. Thus, they are available only on var read-only collections. For mutable collections, they modify the collection if it’s a val. For more details see Collection Write Operations.